Chap, in.] THE OHM. 33 



has increased the intensity by diminishing the resis- 

 tance. If, however, the wires r^ r 2 and r 3 present 

 different resistances, then the quantity of electricity 

 will be different along the three 

 wires, and will be less in the 

 wire of great resistance. In 

 fact, the intensity of the cur- 

 rent in each wire will be in- 

 versely proportional to the re- 

 sistance. The knowledge of this Fig. 18. Divided Circuits. 



fact shows a method by which a 



main circuit may be tapped, as it were, and a portion 

 of the current diverted through some apparatus, a 

 portion regulated by the resistance of the conductor 

 employed. This method will be seen in use in 

 certain physiological experiments to be described. 



The unit of resistance. It is necessary to be 

 able to measure with accuracy the amount of resis- 

 tance that is interposed in the pathway of a voltaic 

 current, and for this purpose a standard of measure is 

 required, just as, to return to the example used in 

 regard to electromotive force (page 17), it is necessary 

 to have a standard of comparison for the purpose of 

 estimating the weight of a body, viz. the pound. 

 Various standards have at various times been used 

 for resistance. Thus one standard proposed was the 

 amount of resistance offered by a copper wire of a par- 

 ticular weight, length, and diameter, and this amount 

 gave the unit of resistance. Another proposal was that 

 of Siemens, in which the unit was the resistance of 

 a column of mercury one metre long, and one square 

 millimetre in section. It should be noted that the 

 resistance in both cases was altered by the tempera- 

 ture of the copper or mercury, therefore the measure- 

 ments were to be taken at C. The unit now 

 employed is usually called the British Association 

 unit, because it was determined by a committee of 

 D 7 



