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The joints between railway rails were, for many years, made with 'fishplates' which were bolted into slots in the ends of a pair of rails, joining them together and holding them in alignment. These joints, which were necessary to allow the rails to expand and contract with temperature changes, were the cause of the familiar 'clickety-clack' sound from train wheels as they ran across the inevitable small gaps between rail ends.

The rails were also called upon to carry small electric currents for signalling purposes but the joints, being subject to rust and movement, could not be relied upon to give good electrical continuity. For this reason, small wires are connected across each joint along a railway track.

In a tramway system, traction currents many orders of magnitude greater than railway signalling currents pass along the rails and any slight resistance in a joint will drop a considerable voltage and generate a great deal of heat. (A 1 ohm joint carrying 200 amps will develop 40 kilowatts of heat, at least enough to raise it to red heat)
Where joints in a tramway are unavoidable, they are bridged by a 'rail-bond', a flexible conductor of large cross-sectional area.

An Expanding Rivet type Rail Bond

The reason for allowing an expansion gap between the ends of sections of conventional railway track, is to prevent the track from being moved by expansion forces. On an extremely hot or cold day, the rail gauge or the alignment of the whole track could be altered with a serious risk of causing a derailment.

Street-running tram tracks, on the other hand, are embedded in the road surface and are not prone to movement. Expansion and contraction forces can be taken up by the natural springiness of the rail material without any movement, provided the fixing method is strong enough.This permits rail lengths to be joined directly to each other by the strongest and most permanent methods, usually welding; ensuring minimal wear, quiet running and low electrical resistance.

The most popular methods of welding rails were electrical resistance welding with hydraulic pressure and 'Thermit' welding using an exothermic chemical reaction to generate molten steel in a crucible at the weld site. Of these, the Thermit process uses less elaborate apparatus and is still the preferred method of welding modern tramway tracks.

Electrical Resistance Welding Outfit	Thermit Welding Process
	Key:   a  Crucible containing 'thermit' reaction   b  Crucible support   c  Clamp holding rail ends in alignment   d  Tapping rod to release molten metal flow into mould

After the rails have been bonded, each joint must be carefully checked for resistance. This was done by passing a current along the track by means of a 'dummy load' and comparing the voltage developed across the joint with that developed across a known length of track. The instrument originally used for this purpose was an electromechanical device of ingenious simplicity.

The Telephonic Bond Tester

The Telephonic Bond Tester picked up the voltages across a lenth of unjointed rail and across the joint on test by means of an insulating wooden strip pressing contacts onto the rail surface The test set comprised a pair of contacts operated by a clockwork motor. The voltages to be compared were alternately switched by these contacts to a pair of head-telephones (headphones). If there was a difference between the voltages, this was heard as a repeated click, each time the contacts operated. For measurement purposes, one of the voltages could be reduced by a potentiometer until it equalled the other and the clicks ceased. The proportion of reduction could be read off a dial and noted for future comparison.

The 'dummy load' which drew the heavy current needed to produce the test voltages was provided by an adapted tram. This was laden with barrels of salt water in which were pairs of metal plates, connected alternately to the overhead wire and to the track. By adjusting the salt content of the water, a heavy current could be made to flow. The resulting heat was dissipated in the water which was allowed to cool between tests.

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