Serial Number 1980970
Bonding Tester Made by Evershed and Vignoles for Royal Air force (part number 5G/6625-99-109-7544) The instrument is housed in a wooden box with sockets for two probes and a compartment for a rechargeable alkaline cell. The meter movement has two deflection coils and is similar to that found in insulation testers made by this firm see: megger though in this case a large current provided by a rechargeable alkaline cell is passed through the circuit to be tested and one deflection coil and the voltage developed across the circuit is applied to the other. The instrument can measure from zero to 0.1 ohm
Colin Ferrier who was an aircraft electrical fitter in the RAF has told me that the bonding tester was used quite frequently to check earth bonding of aircraft, particularly following fitment of new metal parts to aircraft ( ailerons, radio trays etc).
View of interior from below showing probe sockets and battery compartment
View of interior of an earlier model [serial number 1129366]
This instrument covers a wider range of resistance than mine which was intended to be powered by a single NiFe cell. This one part 5G/433 indicates that it is also an ex Air Ministry instrument but an earlier device than mine which has a NATO part number. I believe the metal plates in the right hand compartment are intended to make contact with four single 1.5 volt cells, connected in parallel.
I have found serial numbers for two more similar instruments: 452049 and 867203 [5G 2126] these both have Bakelite sockets.
Note that there are what appear to be low value resistors (about 0.01 ohms) in series with each core of the probe leads. What are they for? I have taken the probe apart to show what is inside. I need another bonding tester to determine the resistance of the tiny ones you see here! I imagine that they are designed to compensate for the change in resistance of the cores of the copper cable with temperature. A considerable current can flow between the two probes which could warm the cables and the coils of the meter movement. Those in the box attached to the rear of the sockets perhaps serve the same function for the meter movements and internal wiring. The lead I made was from a piece of 2 core mains cable soldered to a piece of brass rod filed to a point. What do you reckon?
The two core cable is terminated with low resistance wire wound resistors which are soldered to the substantial probe all housed black Bakelite handle. Unfortunately I have only one probe and have made myself another but without the resistors. Nevertheless I have tried the instrument with an alkaline cell and my homemade probe and have determined that it works.