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.

Safety

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.

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.

Safety

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.

Obtain or construct a wooden box 30 cm high and wide, and 60 cm in length.

1. Fit panes of window glass in the top and front of the box. The glass panes can be held in place with silicone sealant.
2. 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.
3. 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.
4. You can cover the window with different kinds of openings cut from cardboard and fastened with drawing pins.

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.

Safety

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:

© University of York Science Education Group

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:

© University of York Science Education Group

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.

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.

Procedure

1. 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
2. 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.
3. Now centre the trace with the 'X-shift' and 'Y-shift' controls, and adjust' Focus' to give a sharp trace.
4. 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.
5. 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.

The time-base

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:

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.

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.

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.