244 STUDIES IN ELECTRO-PHYSIOLOGY: 



surface of the circular plate makes contact with the centre 

 of the forehead ; the circuit being completed by means of 

 another electrode 



Fig. 188. 



These, preferably, should be three in number ; the 

 boss, a, having diameters of \ in., T % in. and ^ in. respec- 

 tively. 



The readings obtained, as I explain later on, will be in 

 conformity with the hand-to-hand deflection and Ohm's 

 law. 



In the galvanometric diagnosis of morbid conditions 

 the sign of current is of little importance. All the deflec- 

 tions are comparative. The one thing that matters is the 

 quantity of current issuing from any part of the body, and 

 this is shown by the relative rapidity of the excursion of 

 the light upon the scale ; the gradations being from a very 

 rapid off-scale deflection in the case of acute local pyrexia 

 to no deflection at all in cancer. 



For diagnosis I recommend the use of a large head-plate, 

 for the reason that it is imperatively necessary to cover 

 the central line in order to obtain accurate comparison 

 between two symmetrical parts of the body, but in research 

 work, as, for instance, attempting to differentiate efferent 

 from afferent nerves, sign of current is of the utmost 

 consequence, and the head-plate must, therefore, be of 

 exactly the same area and resistance as the electrode used 

 to complete the circuit. 



Formerly I had all these electrodes made of solid silver, 

 but it involved a quite unnecessary "expense. 



