172 PHYSIOLOGY or MCSCI.I:- AND M:I;\ i:s. 



dr:i\vii downward from <i It. In tin- centre there is no 

 tension. At any ]><>int between the middle and the 

 end ", :-ay at <-, a positive tension must prevail which is 

 less than that at <i, but greater thanO. It is expressed 

 Ity the line e f. Similarly at any point between the 

 middle an 1 the end 6, say at //, there is a definite 

 negative tension which may be expressed by the line 

 // It. The same thing may be done for each of the 

 other points in the wire. If the wire is quite uniform, 

 the positive tension decreases quite regularly from tin- 

 end a to the middle, and in the same way the nega- 

 tive tension decreases quite regularly from the end b 

 to the middle. Uniting the ends of the lines which 



FIG. -11. Tin-: FAI.I, IN TIIK KI.KCI UK 1 1\ . 



thus express the tensions, the result is an oblique 

 straight line which cuts the wire in the centre, and 

 the distance of which from the wire at any point re- 

 presents the tension at that point. 



This regular decrease in the tensions prevailing in 

 the wire may be shown by means of an electrometer, if 

 the latter is brought into contact \\Jth each point in 

 the wire. The gradual decrease of the tensions in the 

 wire is e\idently also the essential cau-e of the move- 

 in. -nt of the electricity through the \\ire, for nt each 

 point in the wire there are adjacent portions in which 

 tin- ten-ion- gradually become less from left to right, 

 so that the elect ricity is enabled to How from left to 

 right. The case i- evidently like that of a tube through 



