Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

The current-through-glass demonstration

Demonstration

This rather dangerous but startling experiment shows current being carried through a solid electrolyte. It never fails to 'wow' the students and is well worth the effort of setting it up.

Apparatus and materials

Safety screen

Connector box

Insulating gloves

Bare copper wire, 14 -18 SWG (the wire can be tinned), 20 cm lengths, 2

Soda glass, pieces 15 cm long, 7-12 mm diameter

Electric heater, small

Bunsen burner

Mains lamp (a carbon filament lamp is best, next a clear tungsten filament of 40 W) in a safety batten lamp holder

Clamp stand and bosses, 2

Tripod

Protecting mat

Pliers

Screwdriver

Health & Safety and Technical notes


Experiments involving mains electricity present a hazard to students, teachers and technicians. 
 
The connector box described here offers a safe method of connection. Connections involving mains should never be made with 4 mm plugs and sockets or by twisting wires. 
 
Do this demonstration on a side bench. Use a safety screen to prevent anyone, including the teacher, from inadvertently touching a live contact at mains voltage. 
 
Be sure to isolate the apparatus from mains before investigating any fault. 
 
You should try this demonstration in advance before doing it with a class. 

It is essential that soda glass is used so that there are enough sodium ions in the glass. The sodium ions can drift easily when the rod melts, and so produce a current. 

The electric fire acts as a limiting resistor.

 

Procedure


a Wind the ends of the wire tightly round the glass rod. Three or four turns are best and about 3 cm apart. 

b Make sure the connector box is not connected to the mains. Tighten the other ends of the wires in the connector box terminals. 
 
c Support the glass rod in two clamp stands ensuring no contact with the coils by clamping the board through which the wires pass. Earth the clamp stands. 
 
d Earth the Bunsen burner by attaching the bare end of the green/yellow wire tightly to it. This can sometimes be done by loosening the barrel of the burner, twisting the bare wire round the exposed screw thread and tightening it up again. Alternatively, a terminal could be fitted to the base. Place a heat proof mat below the glass rod to catch any drips. 
 
e Plug the electric fire into one socket on the box and the mains lamp into the other. Make sure the electric heater is in a safe place. This connects the electric fire and mains lamp in parallel with each other but in series with the glass rod. 
 
f Place the box on the tripod and adjust the wires until the glass rod will be in the hottest part of the flame of the Bunsen when this is standing on the bench. 

Light Bunsen and adjust flam for maximum temperaturg Plug the connector box into the mains and switch on. No current flows. 
 
h Place the safety screen between the apparatus and the class. Put on gloves. 
 
i Light the Bunsen and adjust the flame for maximum temperature. 
 
j Turn off the Bunsen when the glass becomes self luminous and the lamp starts to glow. The glass should continue to glow and may eventually melt. 
 
k Failure is usually due to the two wires being too far apart. 
 
NB Before touching the apparatus you must switch off; one of the conductors is likely to be live. 


Teaching notes


1 The sodium ions in the glass carry a current when the glass is fluid enough to let them move. This current through the rod then produces sufficient heating to carry the glass up to melting point and make it drip. This is known as thermal runaway. 

2 At an elementary level, you could ask students to tell you whether glass is a conductor or an insulator. Then show them that the answer depends on its temperature. 
 
At advanced level, you could discuss what provides the charge carriers in the glass. At a high enough temperature, the sodium atoms become mobile and the material loses its stiffness. 
 
This experiment was safety-checked in September 2006