ANIMAL AND VEGETABLE 249 



The potential gradient gives us potential differences, 

 and not the absolute potential at any point. If the cell 

 and circuit be all insulated, the potential at some parts 

 will be -f and at the other parts , depending upon the 

 capacity of the various parts of the circuit. If we connect 

 the circuit with earth at any one point, we have only to 

 draw a line parallel to the base line through the correspond- 

 ing point on the gradient, and perpendiculars to this line 

 will then give the absolute potential, positive when above 

 and negative when below this line. The figures drawn 

 would represent the potential, supposing the zinc plate to 

 be to earth. (Cummings.) 



It is, of course, a matter of extreme difficulty to apply 

 Ohm's law to the human body in the absence of more 

 definite information as to its electrical structure and in 

 view of the changes which occur, even in normal conditions, 

 in its E.M.F., capacity, and resistances ; but I am con- 

 vinced that when the nervous system is studied on electrical 

 as well as chemical lines and in relation to this law, a great 

 advance will be made in our knowledge of the human 

 organism. 



HAND-TO-HAND DEFLECTION. 



In taking the hand-to-hand deflection several pre- 

 cautions are necessary 



(1) The patient should be placed in contact with an 

 " earth " of low resistance for five or, preferably, more, 

 minutes before testing. A copper rod or tube connected 

 by an insulated wire (with a thick conductor) to the water- 

 main makes a very good " earth." 



(2) Rings must be removed from the fingers, as they 

 introduce difference of contact ; and all steel, such as keys 

 and knives, from the pockets, as steel is always more or 

 less magnetic. Gold, silver, and copper coins do not 

 matter. 



