ELECTROMOTIVE FORCE. 167 



thus affording proof that no cause of the generation of 

 currents is present in any part of the apparatus. If the 

 body to be examined is then substituted for the third 

 pad, with the result of deflecting the needle, proof is 

 afforded that some cause effecting the generation of a 

 current exists in the body. The only disadvantage 

 of the arrangement is that the animal substances thus 



O 



examined, being in contact with the concentrated solu- 



* O 



tion of sulphate of zinc, are corroded, and their vital 

 qualities are injured. To avoid this, so-called protec- 

 tive shields, i.e. thin plates of plastic clay (porcelain) 

 which has been mixed with a diluted solution of com- 

 mon salt (i to 1 per cent.), are used. These are placed 

 on the pads of blotting-paper, where the tissue to be 

 examined touches the latter. The clay protects the 

 tissue from direct contact with the solution of sulphate 

 of zinc, though, clay being a conductor, the electric 

 action present in the tissues can reach the zinc and the 

 wires of the multiplier. 



7. In examining muscles or nerves by this method, 

 according to the way in which the animal substance is 

 applied, sometimes no deflection of the magnetic needle 

 is observable, sometimes slight, and sometimes stronger 

 deflections appear. The same body, for example a piece 

 of muscle, may in one position afford a very strong cur- 

 rent, while in another position it affords none at all. 

 In order to understand this, we must examine the way 

 in which the electric currents present within the tissue 

 examined are able to impart themselves to the wire of 

 the multiplier, in the case of the method of experiment, 

 selected. 



Let us revert to the simple apparatus (fig. 36, p. 159) ? 

 in which we first studied the action of electric currents 



