238 STUDIES IN ELECTRO-PHYSIOLOGY: 



satisfactorily within three-quarters of a mile of an electric 

 railway or tube or charging station by reason of the 

 frequent alteration of load. It is true that, as the human 

 body is similarly affected, the argument must also apply to 

 any galvanometer, but in research work one is not always 

 testing the human body, or dealing with such infinitesimal 

 electromotive forces and currents. 



The cost price of this form of Kelvin is about 30. 



We will now consider an instrument of the d'Arsonval 

 type, which, with equal sensibility, can be bought for 

 about 10. 



Fig. 126. 



In this the reflecting mirror does not carry a magnet, 

 but is directly connected with the coil, which, as will be 

 seen, is suspended between the poles of two laminated bar- 

 magnets. At the suspension-head there is a milled pin, by 

 means of which the suspension may be raised or lowered, 

 and a movable head which may be turned one way or the 

 other to adjust the zero. No spirit-levels are provided, but 

 the instrument may be levelled by placing a small spirit- 

 level upon the base as shown in the other instrument 

 and testing it by means of the levelling screws, taking care 

 that the coil swings freely and is equi-distant between the 



