﻿Galvanometer-Needle for Transient Currents. 209 



the internal resistance of the condenser found experimentally 

 by means of the dead-point line was 4 ohms. In addition 

 to the experimental points shown the following were also 

 found : — 



Resistance in ohms ... 



200 



500 



1000 



10000 



100000 



i Throw 



- s 



-10 



-10 



-12 



-12 





The throws on charge were given by 



y= 12 +432/(a+5), 



very approximately. 



Equation (4) can also be written in the form 



^ 1 _(9 o =(l/M/ l :VH) 1 ' 2 KV{±^G + ( 7 V/2Rj^}. 



Hence if we vary V keeping everything else constant and 

 plot a carve showing the relation between the throw and the 

 voltage, we get a parabola. Similarly, if we vary K (keeping 

 R constant) we get a straight line. 



The above tests could not be taken with the greatest 

 accuracy, as the needle-galvanometers were purposely left 

 unshielded so as not to complicate the problem, and the mag- 

 netic effects on the needle of the currents required for a 

 neighbouring tramway were very pronounced. They, however, 

 amply prove that the error made by the assumption that 7 is 

 constant is small. 



The following simple experiment shows the effects pro- 

 duced by a transient current, the integral value of which is 

 zero, on the throw of a neeedle galvanometer. A coil of 

 No. 16 insulated wire, weighing about one pound is placed 

 on the top of a similar coil, the two coils thus forming an 

 air-core transformer. The terminals of one coil are connected 

 with the galvanometer terminals. The terminals of the 

 other are connected by a Morse key with the terminals of a 

 one-microfarad condenser in such a way that the transient 

 currents obtained either by charging the condenser from the 

 100 volt mains or by discharging it pass through the coil. 

 The integral value of the current induced in the first coil in 

 either case will obviously be zero. When the needle is in 

 the symmetrical position the throws produced on charge and 

 discharge are negligibly small, but if the needle be on one 

 side or the other of the symmetrical position the throws will 

 be large and will always be away from the symmetrical 



Phil. May. S. 6. Yol/l2. No. 69. Sept. 1906. ' P 



