MEASUREMENT OF DIFFERENCES OF TENSION. 185 



passed from the apparatus A". W indicates an arrange- 

 ment by which the current of this apparatus may be 

 made to pass as desired either from R to R' or in the 

 opposite direction. T is a multiplier by the deflection 

 of which proof may be obtained that the current of this 

 apparatus remains constant in its strength. The other 

 parts given in the figure we will for the present dis- 

 regard. According to what we have already seen (ch. 

 ix. § 7) a definite electric fall must be present in the 

 rheochord. Let us assume that the current passes from 

 R' to R, that the tension at jR = 0, and that it in- 

 creases toward R\ As the rheochord line is entirely 

 homogeneous, this increase must take place quite regu- 

 larly ; i.e. the tension at every point of the chord must 

 be proportionate to the distance of that point from R» 

 Now let us imagine that a body, A B, within which 

 an electromotive force is present, is to be examined. 

 Naturally two points on its surface, a and b, have dif- 

 ferent tensions, and it is this difierence which is to be 

 measured. The point a must be united by means of a 

 wire (in which is inserted as sensitive a multiplier as 

 possible) with R ; the point b must be connected by a 

 wire with a sliding-piece S which moves on the rheo- 

 chord line. Two differences of tension now act on the 

 multiplier. Firstly, the differences of tension between 

 the points R and S of the rheochord; and, secondly, 

 that between the points a and b. If at b there is a 

 greater positive tension than at a, then the two dif- 

 ferences of tension are opposed in action.^ As the 



• If the positive tension were greater at a than at b, then it 

 would be necessary to reverse the direction of the current within 

 the rhecohord. The commutator W is therefore inserted to effect 

 this reversal of the current. 



