ANIMAL AND VEGETABLE 69 



electrodes, which by bringing conductors nearer to the 

 surface of the skin lower the skin resistance. 



Hand-to-Hand Deflection and Thumb Pressure. The 

 importance of the hand-to-hand deflection, as being the 

 measure of the electromotive force exerted in the body at 

 the time of testing, is fully treated in the chapter upon 

 Ohm's law and electro-diagnosis, but it may serve a useful 

 purpose to explain what happens when there is inequality 

 of pressure of the two thumbs. The body is connected in 

 the galvanometer circuit by means of two suitable metallic 

 electrodes, grasped in the hands, and a certain deflection is 

 obtained. The thumbs carry a greater quantity of 

 current than the fingers, so that if one is pressed harder 

 than the other the deflection is altered, while if one thumb 

 is relaxed and the other pressed down there may even be 

 a reversal of sign, because the direction of current is 

 determined by the path of least resistance. 



Even some electricians of my acquaintance find this 

 difficult to understand. They are accustomed to reason 

 in terms of bare wires, and forget that the wires or con- 

 ductors of the thumbs have an outer coating, or absolute 

 insulation, of 5,000 or more ohms resistance, in the skin. 

 Suppose this resistance to remain unimpaired upon one 

 thumb and even partly removed from the other, and the 

 path of least resistance becomes obvious. If, however, 

 polarity was in the electrodes and not in the hands, no 

 reversal of sign could be brought about by such difference 

 of pressure. 



A simple diagram will explain the differences of thumb 

 pressure. 



Let the body be represented by a source or sources of 

 electrical energy, the arms by two coils of equal resistance, 

 and the thumbs by two variable resistance-boxes, a and b. 

 The quantity of current arriving at points c and d will be 

 exactly equal, because, finding two paths of the same 



