﻿Galvanometer-Needle for Transient Currents. 205 



The deflexion 6 is found by solving the equation 



yLtGr*' cos# + Yi 2 sin #cos 6 = /jlH. sin (0 — 6 ). 



Hence, if the left-hand side of this equation be zero, 

 = O will be a position of equilibrium and there will 

 therefore be no deflexion. In this case 



n(±l<y— —i sin 0o> 



In many types of galvanometer the neutral position for a 

 given current can be found very easily, and hence this 

 equation can be used to give us the ratio of /xGr to 7. 



The Dead Points of the Needle for Transient Currents. 



Lord Rayleigh, in a paper to the British Association in 

 1883*, points out that a galvanometer is a very imperfect 

 instrument for indicating whether the integral sum of the 

 transient currents through it is zero or not, and he also 

 mentions the limitations that this imposes on Maxwell's 

 method of comparing mutual inductances. The author finds 

 that with many needle-galvanometers it is easy to arrange so 

 that a, relatively speaking, gigantic charge can be passed 

 through the coil without producing any throw at all. He 

 also finds that all the galvanometers he has tested, whether 

 needle or moving coil, will produce throws with certain 

 transient currents, although their integral values are zero. 

 A simple explanation of the effects produced by a condenser 

 discharge can be given as follows. 



Let us consider the case of a condenser K connected with 

 the galvanometer coil in the usual manner by a charge and 

 discharge key. Let ^ be the initial charge in the condenser, 

 then we have 



and 



r* CO 



idt = q 



Rp 2 ^ = 2o 2 /(2K)--W; 

 Jo 



where R is the effective resistance of the discharging circuit 

 and W the total energy given to the needle. The latter 

 equation is very approximately true whether the discharge be 

 oscillatory or not. If we assume that the current has become 

 negligibly small before the needle has moved appreciably, we 



* Lord Rayleigh, Brit. Assoc. Reports, p. 444 (1883), " On the Im- 

 — a test of the E— 

 vol. ii. p. 228. 



* -Liora xiayieig-n, x>rii. assoc. _. 

 perfection of the Galvanometer as a test of the Evanescence of a Transient 

 Current " ; or' Scientific Papers,' 



