238 



TWO IMPORTANT 



obtained when a single individual, whether man, horse, or dog, is 

 connected with the electrical indicator. 



Fig. I. 

 The principles of an electrical indicator may be understood by 

 reference to the diagrams, in which the effects of water-pressure 

 are compared with those of electrical pressure or potential. A 

 and B, Fig. i, are two bottles of water, each connected by flexible 

 pipes with a bent tube half full of mercury. If the two bottles are 

 at the same level, the mercury in the bent tube remains at zero, 

 and it is evident that this is still the case if both bottles be raised 

 together or lowered together. But if the bottles be moved un- 

 equally, either up or down, the level of the mercury will alter. If 

 A is lower than B, the mercury in this limb of the tube will move 

 upwards, whereas if B is lower than A, it will move downwards. 

 And if we imagine everything hidden from us by a screen, except 

 a portion of the tube, whilst the two bottles are being moved by 

 unseen hands, it is obvious we shall be able to tell by the move- 

 ments of the index whether A is below B, or B below A. 



Fig. 2. 

 Let our instrument now be applied to the heart. This, which 



