198 INTRODUCTION TO GENERAL PHYSIOLOGY 



with 0.7 per cent, sodium chloride if the electrodes are to be 

 used on frog's tissue, or 0.9 per cent, if for mammals. This paste 

 is shaken up with a larger quantity of the solution and poured 

 into the bottle so as nearly to fill it. A piece of cotton spirit 

 lamp wick has previously been pushed into the bent tube, and the 

 solution is run in until it escapes from the end of this tube. The 

 clip is then closed. The wick may be cut to any size or shape, 

 especially if a little kaolin is put on it, according to the organ or 

 tissue to be investigated. Two of these electrodes will be needed. 

 It will be found that when any two points on a living tissue are 



FlG. 17. Circuit for Experiments on Electrical Changes. 



A B, slide-wire. 



C, battery. 



D, key. 



E, electrodes. 



F, tissue. 



G, galvanometer. 



connected to the galvanometer there is usually a deflection, which 

 may be great enough to send the spot of light off the scale. This 

 is partly due to differences of potential in the tissue itself, partly 

 to unavoidable inequalities in the electrodes. To diminish the 

 latter as far as possible, it is well to keep the electrodes connected 

 together when not in use. This is done by placing a wire with 

 its ends one in each of the mercury tubes and connecting the bent 

 tubes with a piece of india-rubber tubing filled with salt solution. 



But in any case a means of balancing the electro-motive force 

 present in the tissue is necessary. An equal and opposite electro- 

 motive force is put into the circuit by an adjustable contact on a wire 

 through which a current is flowing. The slide-wire used previously 

 will serve. The whole circuit is arranged as in Fig. 17. 



It is convenient to have an adjustable resistance connected across 



