Prof. Ayrton and Mr. Mather on Galvanometers. 367 



of the motion is large or small. Since quickening the move* 

 ment by increasing the control (?'. e. decreasing the periodic 

 time of vibration) of a given system lessens the decrement, 

 the two uses of the words " dead-beat " are to some extent 

 incompatible. Maxwell used the term for galvanometers in 

 which the motion of the suspended system was u aperiodic/' 

 i. e. the system only passed once through the position of 

 equilibrium before coming to rest. As this meaning of the 

 word seems most rational, we think it desirable it should be 

 retained, and that its use in connexion with instruments having 

 rapidly moving systems w T hose motion is not aperiodic should 

 be discontinued. The expressions " quick-moving " or " short- 

 period " galvanometer might be used to denote this class of 

 instrument. 



Insulation of Coils and Terminals of Galvanometers, and the 

 use of Price's " Guard- Wire/' 

 When testing very high resistances, such as the insulation 

 resistance of a short length of good cable, with a galvano- 

 meter, the difficulties which arise from leakage over the cable 

 ends or from the galvanometer itself are well known ; the 

 importance of constructing galvanometers so as to ensure 

 good insulation was referred to in our previous paper *. 



Since that date Mr. W. A. Price f has suggested the use 

 of a l{ guard- wire " which practically eliminates error due to 

 surface leakage over cable ends. The method there described 

 can be adapted to galvanometers, thereby making the question 

 of perfect insulation of coils and terminals of far less conse- 

 quence than formerly. 



Fig. 1. r _ 



=1 ^ J 



TANK 



Mr. Price's method consists in winding a bare wire W 

 about midway between the extremities of the pared ends 

 of the cable as shown in fig. 1, and connecting the other 



* See p. 71, Phil. Mag. July 1890. 



f ' Electrical Review,' vol. xxxvii. p. 702 (1895) 

 on lt Dielectrics," Proc. Phv: 

 error is evaluated. 



See also Appleyard 

 Soc. vol. xiv. pp. 257, 264, where possible 



2D 2 



