Collection General guidance on apparatus
- Carbon dioxide cylinder (syphon type)
- Electric motor, fractional horsepower
- Hand stroboscope
- How to make a smoke box
- Iron filings in a pepper pot
- Infra-red receiver
- Infra-red transmitter
- Jelly 'fibre'
- Operating instructions for a simple oscilloscope
- Power supplies for electromagnetism practicals
- Protactinium generator
- Types of electron tube
- Using an electroscope
- Using wave simulations
- Van de Graaff generator
- Xenon stroboscope
General guidance on apparatus
Teaching and safety notes on a wide range of apparatus used in school/college-based physics practical.
Teaching Guidance for 14-16
Dry ice has many uses. As well as simply watching it sublimate, you could also use it for cloud chambers, dry ice pucks, cooling thermistors and metal wire resistors in resistance experiments, and experiments related to the gas laws. Don't be tempted to get a small cylinder, it will run out too quickly.
What type of cylinder, where do I get CO2 and what will it cost?
A CO2 gas cylinder should be fitted with a dip tube (this is also called a ‘syphon type’ cylinder). This enables you to extract from the cylinder bottom so that you get CO2 in its liquid form, not the vapour.
NOTE: A plain black finish to the cylinder indicates that it will supply vapour from above the liquid. A cylinder with two white stripes, diametrically opposite, indicates it has a syphon tube and is suitable for making dry ice.
A cylinder from British Oxygen will cost about £80 per year for cylinder hire and about £40 each time you need to get it filled up. (The refill charge can be reduced by having your chemistry department cylinders filled up at the same time.)
If the school has its own CO2 cylinder there will be no hire charge, but you will need to have it checked from time to time (along with fire extinguisher checks). Your local fire station or their suppliers may prove a good source for refills.
CLEAPSS leaflet PS45 Refilling CO2 cylinders provides a list of suppliers of CO2.
A dry ice attachment for the cylinder
Dry ice can be made using an attachment that fits directly onto a carbon dioxide cylinder with a syphon tube. Section 13.3.1 of the CLEAPSS Laboratory Handbook explains the use of this attachment (sometimes called Snowpacks or Jetfreezers).
You can buy a Snowpack dry ice maker from Scientific and Chemical. The product number is GFT 070 010
VWR International sells Snowpacks through its UK distributor. The version that makes 30 g pellets of dry ice is catalogue number 3285042/02.
Philip Harris sells similar products. See their 2004 catalogue, page 679.
Remember to wear insulating gloves when handling dry ice.
Arrange good ventilation in the room where it is made and used, and make sure that the gas does not collect at a low point.
Electric motor, fractional horsepower
Unfortunately, this type of motor is no longer made: it has been replaced with an AC induction motor. However, many secondary schools have one of the older type which can be used for this demonstration.
These motors have separate power connections: one for the rotor (armature) and one for the field coils, usually via 4-mm sockets. Details of the electric shock hazard are given in CLEAPSS Bulletin 114, summer 2002.
This label can be printed for attachment to the motor:
Safety: Danger of electric shock
High voltages can be developed at the terminals of this motor. All connections must be made using leads with sprung shrouded plugs. Switch off all power supplies before altering connections. For use by staff and supervised sixth-form students only.
The hand stroboscope is a disc of hardboard or card with a simple pivot at its centre, so that the disc can be kept spinning by hand.
The disc has a finger hole, off-centre, to enable the user to keep it spinning. It has narrow slits on its face, near the rim. The slits are evenly spaced and 12 slits are best. When fewer are needed, you can obscure some slits by sticking black tape over them and use 6, 4, 3, 2 slits, or just 1 slit.
This simple stroboscope enables students to 'freeze' repetitive motions – or to slow them down for closer study. For example, continuous ripples are easier to see by using a stroboscope, especially those ripples with higher frequencies. By viewing a vibrating object through the slits, students can calculate the frequency of a vibration.
The stroboscopes are less likely to judder while rotating if the bearing is not too tight and the handle is held loosely.
Photo-induced epilepsy only accounts for 1% of epileptic attacks, but it is the frequency of flashing rather than the light intensity which causes it. For this reason, a safety note about photo-induced epilepsy is given whenever any type of stroboscope is used.
How to make a smoke box
Obtain or construct a wooden box 30 cm high and wide, and 60 cm in length.
- Fit panes of window glass in the top and front of the box. The glass panes can be held in place with silicone sealant.
- Leave the back open and cover with loosely hung black cloth that drapes like a curtain. Hang this curtain in two sections, with a 10 cm overlap at the centre of the box. Paint the inside of the box with mat black paint.
- Cut a window 10 cm high and 5 cm wide midway between the top and bottom of one end, and 8 cm or 10 cm from the glass front. This window lets in light rays.
- You can cover the window with different kinds of openings cut from cardboard and fastened with drawing pins.
Iron filings in a pepper pot
The use of iron filings
- The filings should not be 'dust' but should have one dimension longer than the other two.
- The 'pepper pot' shaker must have holes large enough to allow the filings of this size to pass through. It can be improvised from a film can with holes through the plastic lid.
- Iron filings can ruin sinks. It is wise to cover all sinks in the lab before issuing iron filings.
Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.
To make the infrared receiver you need:
- infrared phototransistor
- linear miniature pre-set potentiometer, 100 kW, (vertical mount) (Rapid 67-0315)
- 0.1 pitch stripboard
- 3 mm LED clip (JPR 850-090)
- socket 4 mm red (JPR 705-205)
- socket 4 mm black (JPR 705-206)
- halved square section 'plastic down pipe' approx 9 cm long (DIY store)
- connecting wire
Assemble as shown in the diagram:
To make the infrared transmitter you will need:
- 75W resistor (from Rapid metal film resistor MR25 kit. 13-0270)
- infrared-emitting diode SFH487
- 3 mm LED clip (JPR 850-090)
- socket 4 mm red (JPR 705-205)
- socket 4 mm black (JPR 705-206)
- halved square section 'plastic down pipe' approx 9 cm long (DIY store)
- connecting wire
Assemble as shown in the diagram:
To make approx 22 cm long jelly fibre you need:
- mould approx 2 cm x 2 cm x 24 cm (can be made from a length of electrical trunking with pieces of plastic credit card glued across the ends)
- cling film to line mould
- 15 g powdered gelatine (preferably not the ‘veggie’ sort as it does not set well)
- 150 ml water
Mix the gelatine according to the instructions on the packet and pour into the lined mould.
When set, use the cling film to lift the jelly ‘fibre’ from the mould.
Operating instructions for a simple oscilloscope
Teaching Guidance for 14-16
A CRO (cathode ray oscilloscope) is a good voltmeter. The electron stream from the electron gun obeys instructions to move up or down almost instantaneously. The input resistance of the device is very high so that very little current is taken from the circuit under investigation. Voltmeters take a trickle of current.
The spot moves sideways when the time-base is switched on, and the trace can be looked on as the x-axis or time axis of a graph. The signal is applied, normally, across the y-terminals, and so the variation in the potential difference (p.d.) is plotted along the y-axis. The screen plots out the 'graph' of p.d. against time.
- To prepare the oscilloscope for use, plug into the mains supply and set the controls as follows:
- Brightness to OFF
- Focus to the mid position
- X-gain fully anticlockwise
- X-shift to the mid position
- Trig control to +
- Time-base: time/cm (or time/div) control to 1 ms
- Time-base: variable control fully clockwise
- Stability control fully clockwise
- Trig level control fully clockwise
- Amplifier: volts/cm control to 0.5 V/cm
- Y-shift to the mid-position
- Input switch to DC
- Switch on by means of the 'Brightness' control. After warming up for about one minute, turn 'Brightness' clockwise until a trace appears. Set the control so that the trace is clearly visible, but not excessively bright.
- If no trace appears, leave the 'Brightness' in the fully clockwise position, and adjust 'X-shift' and 'Y-shift' until the trace appears. This is best done by rotating 'X-shift' backwards and forwards, whilst slowly advancing 'Y-shift' from the fully anticlockwise position. Immediately the trace is found, reduce 'Brightness' to a convenient level.
- Now centre the trace with the 'X-shift' and 'Y-shift' controls, and adjust' Focus' to give a sharp trace.
- Slowly turn 'Stability' anticlockwise until the trace just disappears. Finally, rotate 'Trig Level' anticlockwise and switch it to the auto position. The trace (which reappears when 'Trig Level' is rotated) may dim when this is done, but will brighten again when an input is applied.
- The oscilloscope is now ready for use, but it is important to be familiar with the function of the various controls. This experience is best gained by a 50 Hz wave-form from an AC low-voltage power supply, and then exploring the action of the various controls (excepting 'Stability' and 'Trig Level' controls which are set by procedure d above). A possible routine for those unfamiliar with such instruments is to:
- put 2 to 4 volts, 50 Hz AC on the input and change volts/cm back to 5
- turn the variable time-base control fully anticlockwise and then back to the calibrated position (fully clockwise)
- change time-base to 100 μs, and then return it to 1 ms
- change Trig + to - (if the sine curve trace is not inverted by this, turn the Stability control very slightly anticlockwise until it is).
Further work should bring increasing confidence.
For most experiments, the 'Trig Level' can be left at AUTO. To give a steady trace, the 'Stability' should be turned as far as possible counter-clockwise without losing the trace. This setting may vary a little with different time-base speeds.
Further details on the oscilloscope
AC – DC switch The switch should normally be set to DC, even when the oscilloscope is used for AC. In the AC position there is a capacitor in series with the input and this will separate the AC component from a wave-form such as the one in the sketch. The AC position of the switch should be used only for this purpose.
When the oscilloscope is used for the pure AC, setting the switch to AC will cause a smaller deflection at very small frequencies because C and R modify the signal fed to the tube. This is another reason for not using it except for the purpose indicated above.
Time base When time-base is switched off the spot is automatically centred. There is no X-shift control.
When the time-base is switched on, the speed of the spot is determined by the setting of the ‘Range’ and ‘Variable’ controls. However, the frequency of repetition of the time-base is not much increased at the higher speeds and the time-base is often interrupted by slow changes of the input voltages. For these reasons it is better to have the time-base off when the oscilloscope is being used as a DC voltmeter.
When an alternating voltage is connected to the input, it automatically triggers the time-base and makes it a very steady trace.
X-input The time-base should first be switched off by turning the 'Variable' control fully anti-clockwise to the OFF position. AC inputs may then be connected to the X-input and sockets on the back of the oscilloscope. (Note. The socket on the back and the E terminal on the front are connected internally.)
The 'X-gain' control will give a variation of 2:1 in the amplification. The spot will be deflected horizontally to the full screen width by AC voltages between 3 V r.m.s. and 6 V r.m.s. The sensitivity varies from 2 V/cm to 1 V/cm.
(There is no direct coupling between the X-input socket and the cathode ray tube. DC inputs will give only momentary deflections.)
Z-input If a sine wave or square wave input is connected between the Z-input and Earth sockets on the back of the oscilloscope, the brightness of the trace may be varied by these inputs.
With sine wave inputs, at least 20 V r.m.s. is needed at a frequency of 50 Hz. This reduces to 1 V r.m.s. at 20 kHz. It is necessary to dim the trace, so that the variation in brightness may be easily noticed.
With square-wave inputs, 30 V peak to peak is necessary at 50 Hz. This reduces to 2 V peak to peak at 20 kHz. The variation in brightness is much clearer with square waves. With low frequencies, quite sudden increases or decreases in brilliance can be seen.
Sweep output When the time-base is switched on, a p.d. corresponding to the X deflection may be taken from the 'sweep output' and Earth terminals. The potential of the 'sweep output' terminal varies from about +40 V, when the trace is on the left of the tube, to about +20 V when the trace is on the right. Too much current should not be taken, unless distortion of the time-base is permissible.
As a rough guide, the time-base will not be affected if a 0.1 μF capacitor (to block the DC component) is connected in series with the sweep output, and the load resistance is not less than 100 kΩ. At some sweep speeds, much more current may be taken.
It is easiest to see if the load circuit is distorting the time-base by unplugging it momentarily.
The sweep output may be used for triggering any transient effect repeatedly so that a steady pattern occurs on the tube.
It is also interesting, and helps to understand the operation of the time-base, to connect the sweep output of one oscilloscope to the Y-input of a second oscilloscope.
A video showing how to use an oscilloscope:
Power supplies for electromagnetism practicals
Teaching Guidance for 11-14
Very-low-voltage 'Westminster pattern' supplies are best.
When a wire or coil of thick wire is connected across the terminals of a low-voltage power supply it effectively provides a short circuit.
Most general purpose power supplies are designed so that, under these conditions, an output circuit-breaker operates to switch off the unit. This protects the transformer and rectifiers from damage due to overheating.
The best solution to this problem is to have a special, very-low-voltage power supply for electromagnetism (the Westminster pattern). This has a special transformer with a few, very thick turns as its secondary. The maximum AC voltage is 2 V rms, with a 1 V centre tap. High current rectifier diodes are used to give full-wave rectified DC output at 1 V.
When any of these outputs are shorted, the current flow in the transformer secondary is about 8 A. Most of the power is dissipated in the external circuit, so that the transformer is undamaged. However, the external circuit will become hot.
If a general-purpose power supply is used, even when set to 1 or 2 V AC or DC, the temperature rise in the transformer may damage it or, at best, cause a fuse to operate which can only be replaced by an expert.
Teaching Guidance for 14-16
When using the protactinium generator you will also need a:
- Small polypropylene bottle (30 ml capacity)
- Separating funnel or beaker
- Uranyl nitrate (or uranium oxide dissolved in nitric acid)
- Concentrated hydrochloric acid, 7 ml
- Iso-butyl methyl ketone, or amyl acetate
- Tray lined with absorbent paper
The chemistry of the protactinium generator
The first steps of the uranium-238 series are involved in this experiment.
The aqueous solution (at the bottom of the bottle) contains the uranium-238, its daughter thorium-234 and the short-lived granddaughter protactinium-234.
Uranium and protactinium both form anionic chloride complexes but thorium does not. At high hydrogen ion concentrations, these complexes will dissolve in the organic layer (which is floating on top of the aqueous solution).
When you shake the bottle, about 95% of the short-lived granddaughter (protactinium) and some of the uranium will be dissolved in the organic layer. The thorium stays in the aqueous layer.
Radioactivity is a property of the innermost nucleus of the atom, so it is not affected by chemical combination.
The granddaughter (in the organic layer) decays without any more being produced by its parent (thorium), all of which is still in the aqueous layer. It emits beta particles, which travel through the plastic wall of the bottle. Isolating the protactinium in the top (organic) layer allows it to decay without any top-up from its parent (thorium).
The radiation from the thorium and uranium should not interfere with the results, for two reasons:
• The counter does not detect the alpha particles from the uranium or the low-energy beta particles from the thorium; it only records the high-energy (2 MeV) beta particles from the granddaughter (protactinium).
• The uranium-238 decays with an extremely long half-life. It yields a meagre, almost constant, stream of low-energy alpha particles. Its daughter, thorium-234, decays with a half-life of 24 days. During the length of this experiment the decay rate can be assumed to be constant. If these two isotopes contribute to the count at all, it will be accommodated in the background count. The stockpile of thoruim is also constantly topped up in the aqueous layer as long as the protactinium is present with the thoruim.
Types of electron tube
There are a number of different cathode ray tubes available to schools. They all use similar electron guns but have different arrangements within the tube. Each one can be used to illustrate or measure slightly different behaviours of electrons. Some of them can be used for a number of different demonstrations. Also, some effects can be demonstrated using more than one tube. Often, your choice of tube will be determined by what you already have available in your school or college.
Here follows a quick overview of each type of tube and what it is best used for.
1 Fine beam tubes
There are two main types of fine beam tube.
a Leybold style tube
These were made in Germany and have a single electron beam. The path of the electrons shows up blue because there is a residual amount of hydrogen gas in the tube. The magnetic field coils are larger than the tube and normally fixed to the base board.
This can be used for basic deflection experiments and e/m measurements. However, a Teltron tube is better adapted for making the beam go in a complete orbit.
b Teltron tube
This second type of tube is made in the UK by the scientific products supplier 3B Scientific (previously manufactured by Teltron). It has two electron beams, so that one beam fires out across the tube and the other one, at right angles to the first beam, up to the top of the tube. The beam is selected using a switch close to the cathode. The paths of the electron beams are green, because the electrons are travelling through a residual amount of helium gas.
Just outside each gun muzzle there is a pair of plates for deflecting the beam by an electric field. One plate of each pair is attached directly to the gun muzzle which supports it. The other plate of each pair is connected inside the tube to the second side terminal on the tube.
These tubes are useful for e/m measurements because, using the vertical gun, it is possible to get the electron beam to go in a closed orbit.
If the beam fails to make a clear spot then try a small potential difference to the deflecting plates. Another trick is to clean the accumulated charges off the screen by sweeping the beam up and down it and across it.
2 Maltese cross tube
The Maltese cross tube is used to show the shadow produced by a piece of metal in the path of an electron beam. The electron gun is similar to other tubes except that the beam is allowed to spread. The metal cross inside the tube casts a shadow on the fluorescent screen.
3 Deflection tube
The beam from the deflection tube is produced by a horizontal slit in the anode. So the beam fans out to produce a ‘V’ of electrons in the horizontal plane. This is aimed at a vertical fluorescent screen inside the tube. The vertical screen is at an angle to the beam direction. So the fan of electrons cuts across the screen, producing a straight line along it.
The deflection plates are positioned above and below the screen, which has its own graduated scale. So the effect of the deflecting voltage can be measured on the scale.
Using the graticule, it is possible to show that the path is parabolic in an electric field and circular in a magnetic field.
The Perrin tube
This tube has a collecting plate and terminal slightly off-axis at the target end of the tube. This is to allow you to deflect the beam and collect electrons. It is possible to show that the collected charge is negative.
Using an electroscope
A gold leaf electroscope measures potential difference between the leaf and the base (or earth).
The leaf rises because it is repelled by the stem (support). The leaf and its support have the same type of charge. A typical school electroscope will show a deflection for a charge as small as 0.01 pC (the unit pC is a pico coulomb, 1 × 10-12 coulombs, equivalent to the charge on over 6 million electrons).
Charging an electroscope
There are a number of ways of charging an electroscope. They include:
Charging by contact. Rub an insulator to charge it up. Then stroke it across the top plate of the electroscope. This will transfer charge from the insulator to the electroscope. This method is direct and clear to students. However, the charge left on the electroscope will not always leave it fully deflected.
Charging by induction. This is a quick way to get a larger charge onto the electroscope. However, it can look a bit magical to students. So it should be used with some care.
Rub an insulator to charge it up. Bring it close to the top plate of the electroscope – but don’t let it touch. This will induce the opposite charge on the plate of electroscope leaving a net charge on the gold leaf, which will rise. Now touch the plate with you finger momentarily to earth it (still holding the charged insulator near the top plate). The charge on the top plate will be neutralised but there will still be a charge on the gold leaf. Let go of the plate and then take the charged insulator away. The charge that had been pushed down to the gold leaf will now redistribute itself over the plate and the leaf, leaving the whole thing charged. The leaf will show a good deflection.
Charging with an EHT or Van de Graaff generator. You can use a flying lead connected to one of these high voltage sources to charge up the gold leaf electroscope. This is quick, effective and obvious to students. The other terminal of the supply should be earthed. Connect the flying lead to the supply through a safety resistor.
Detecting small currents
The electroscope can be used to demonstrate that a small current is flowing in a circuit – for example in experiments to show the ionisation of the air.
Using the hook rather than the plate makes the electroscope more sensitive to small amounts of charge. A charge of around 0.01 pC will cause a noticeable deflection of the gold leaf. So it is possible to watch it rise (or fall) slowly due to a current as small as 1 pA.
Put the electroscope in series (as though it were an ammeter). Any charge that flows in the circuit will move onto the electroscope making the gold leaf rise. You may need to discharge the electroscope when you first switch on the power supply because there will be an initial movement of charge due to the capacitance in the circuit.
Alternatively, you can use the electroscope as a source of charge and watch it discharge. It is like a capacitor with its own display. Charge it up and then connect it into a circuit. If the circuit conducts, the electroscope (capacitor) will discharge and, at the same time, the leaf will display how much charge is left.
Using the electroscope as a voltmeter or electrometer
The electroscope has a very high (as good as infinite) resistance. If you earth the electroscope case, the electroscope measures potential so it is well suited to detecting potentials in electrostatic experiments. Without earthing, the quantity it is measuring is charge. This is related to p.d. (by its capacitance C , i.e. V = Q/C ). But it isn’t the same as p.d. because the capacitance can vary a lot – even during an experiment. Capacitance depends on the position of the electroscope, people nearby and so on.
So although the electroscope is useful as an indication of a voltage, it isn’t a reliable means of measuring it.
School electroscopes are open to the air (more refined ones are in a vacuum). Cosmic radiation will ionise this air and cause a small leakage current. So the electroscope will discharge over time. Historically, the discharging of electroscopes led to the suggestion of the existence of cosmic radiation. Victor Hess and Carl Anderson shared the Nobel Prize for Physics in 1936, for discoveries related to cosmic radiation. The Nobel Award ceremony speech describes their work:
Using wave simulations
There are many excellent applets available online that show wave behaviour as if observing a ripple tank or oscilloscope screen.
These cannot substitute for experience of the phenomena themselves but provide a powerful way of helping students to visualize. They provide a valuable complement to experiments by removing extraneous effects.
Van de Graaff generator
Van de Graaff generators have a reputation in schools for being “temperamental” in operation. The usual reasons for their failure to perform (assuming the mechanical drive is fine) will be impairment of the belt’s insulation properties and/or drainage of charge to Earth across the surfaces of the pillar support.
Good housekeeping and repairs
Charged surfaces inevitably attract air-borne grime to build up a conduction path to Earth. So it is important to ensure all surfaces are clean and free from moisture before start-up. A convenient and excellent solvent is Swan Lighter fluid – a volatile, light hydrocarbon usable on all plastics likely to be encountered. Dampen a wad of soft tissue and apply lightly to both surfaces of the moving belt, and then wipe the support column thoroughly. (It is also possible to wash the belt in washing up liquid and then dry it thoroughly before use.)
If the cleaned and re-assembled generator does not immediately charge, play hot air from a hair drier on to the moving belt, pillar and dome – maybe for as long as 2 or 3 minutes.
It is not necessary to have the collecting sphere in place to check the onset of charging; use a neon indicator held near the top comb. An alternative is to use the “head of hair” accessory. Once charging starts leave the machine running, playing warm air on the belt for a while; then reassemble with a spark gap of 5-10 mm. Once sparks pass fairly rapidly, increase the gap.
It is also important to keep the outer surface of the collecting sphere clean and unscratched for the production of “fat”, luminous sparks. Otherwise you will get nothing more than corona discharges (characterized by the frying-noise, lack of luminosity and production of detectable amounts of nitrogen oxides) at the sharp points presented by dust particles or surface imperfections.
Keeping a generator in a warm cupboard overnight in preparation for next day’s teaching may be to no avail unless the cleaning has been done first.
In partnership with STEM Learning, the IOP have created this video for teachers to show how to get the best out of a Van de Graaff generator. Michael de Podesta explains how the generator works and gives some tips on getting consistently good results when using the apparatus. The video concludes with a simple but effective demonstration of charge.
Safety: Photo-induced epilepsy
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.