368 Prof. Ayrton and Mr. Mather on Galvanometers. 



end of the wire to H, the battery side of the galvano- 

 meter G. 



As the current through the galvanometer is very small, the 

 P.D. between the wire W and the conductor C is also very 

 small, and the surface leakage between them is quite negligible 

 if the surface of the dielectric between them is fairly clean 

 and dry. The full P.D. exists between W and the sheathing 

 or wet braiding S, but any leakage-current from one to the 

 other does not pass through the galvanometer. In the same 

 manner error due to leakage from the terminal J of the 

 galvanometer may be eliminated. 



It will be evident that there is no need to have the battery 

 side of the galvanometer perfectly insulated : all that is neces- 

 sary is that the insulation resistance of H from earth is large 

 compared with the internal resistance of the battery. 



Probably the simplest way of applying the " guard-wire " 

 principle to a galvanometer is to put one terminal of the 

 winding to frame and case, preferably of metal, and carry the 

 other terminal from the frame on a support moderately well 

 insulated therefrom. The whole galvanometer may be suffi- 

 ciently well insulated from earth by putting short ebonite 

 sleeves, or caps, on the tips of the levelling screws. This 

 construction was carried out in the Ayrton-Mather galvano- 

 meter as made by Mr. R. W. Paul in 1892, an instrument of 

 this class being shown before this Society on June 10th in 

 that year *. By using the frame terminal as the battery side 

 of the galvanometer, the frame acts as an efficient guard-wire. 

 Another advantage of the construction here described is that 

 electrostatic deflexion of the moving system is at the same 

 time prevented, a matter of considerable importance when 

 resistance tests with fairly large potential-differences are 

 being made. 



A similar construction is, of course, applicable to shunt- 

 boxes. 



In concluding this section of our paper, we desire to thank 

 the various manufacturers and inventors wdio have kindly 

 supplied us with information and data relating to their instru- 

 ments, and also to acknowledge our great indebtedness to 

 C. G. Lamb, Esq., M.A., of the Cambridge Engineering 

 Laboratory, and W. R. Cooper, Esq., M.A., for supplying us 

 with several important records by which we have been able to 

 add materially to the completeness of our lists. 



* ' Electrician/ vol. xxix. June 17th, 1892, p. 174. 



