284 STUDIES IN ELECTRO-PHYSIOLOGY: 



CHAPTER XVII 

 ELECTRO DIAGNOSIS 



THE GALVANOMETER AND ELECTRODES 



AND How TO USE THEM 



THE chief requirements in a galvanometer are great 

 sensibility and perfect insulation combined with a short 

 period of oscillation. There are several types, but in 

 practice I prefer for research work the special form of 

 Kelvin reflecting Astatic, made for me by Elliott Bros., 

 although it is somewhat expensive. This instrument is 

 designed for tests where specially good insulation of all 

 parts of the circuit is required. There are eight coils, 

 having a total resistance of from 60,000 to 100,000 ohms, 

 carried in hinged frames supported by ebonite pillars ; 

 four terminals carried on tall ebonite stems through the 

 top of the case, and a long suspension. 



The medical practitioner will be quite safe, as regards 

 sensibility, in ordering an instrument which will give a 

 deflection of 4,000 or more mm., at a scale distance of 

 1 metre, per micro-ampere. The period should not be 

 more than seven seconds. 



On the next page will be found an illustration of the 

 instrument I have mentioned. 



As shown it is not adjusted. To do this it is necessary 

 that it should be placed in the east (facing west), looking 

 towards the scale which is from 1 metre to 41 in. distant. 

 If it is stood upon wood the levelling screws should rest in 



