'34 



CHAPTER XIII. 



RESISTANCES AND THEIR MEASUREMENT. 



THE measurement of the conducting power, i.e. the 

 conductivity, or, what is practically the same thing, the 

 measurement of the resistance, of various bodies, and 

 especially of the various animal tissues, is a subject of 

 growing importance in physiology and in therapeutics. 

 It is of importance in therapeutics because the em- 

 ployment of electricity in the treatment of disease is 

 daily being extended, and its proper application 

 depends upon an appreciation of just such points as 

 the resistance that different tissues offer to the passage 

 of the current. 



It has been seen also (page 27) that the strength 

 of a current is, to a large extent, dependent on the 

 resistance, and that by varying the resistance the 

 current strength may be varied, while by increasing the 

 resistance the current may be weakened^ and vice 

 versa. It has been also noticed how, on this principle, 

 the rheocord (chapter viii. ) is so constructed as to per- 

 mit a very great extent of graduation in the strength 

 of tjie current, and how, on the same principle, the 

 compensator (long or round) affoi'ds a similar means, 

 though to a much more limited extent. On page 33 

 it has been pointed out that there is a standard of 

 resistance as there is a standard of weight, that a coil 

 of fine wire may be prepared, which, at a given tempera- 

 ture, will offer the standard resistance of one ohm, and 

 that by means of this standard other resistances may 

 be compared. It will, further, be readily understood 

 that by means of the standard resistance various ap- 

 paratus may be constructed, other than that of the 



