CHAP, vi.] ELECTRICITY AND MAGNETISM. 325 



given as LT~ l , which are the same dimensions (see Art. 258) as 

 those of a velocity. Every resistance is capable of being 

 expressed as a velocity. The following considerations may 

 assist the student in forming a physical conception of this : 

 Suppose we have a circuit composed of two horizontal rails 



Fig- 134- 



(Fig. 134), CS and DT, I centim. apart, joined at CD, and 

 completed by means of a sliding piece AB. Let this variable 

 circuit be placed in a uniform magnetic field of unit intensity, 

 the lines of force being directed vertically downwards through 

 the circuit. If, now, the slider be moved along towards ST 

 with a velocity of n centimetres per second, the number of 

 additional lines of force embraced by the circuit will increase at 

 the rate n per second ; or, in other words, there will be an 

 induced electromotive - force (Art. 394) impressed upon the 

 circuit, which will cause a current to flow through the slider 

 from A to B. Let the rails have no resistance, then the 

 strength of the current will depend on the resistance of AB. 

 Now let AB move at such a rate that the current shall be of 

 unit strength. If its resistance be one "absolute" (electro- 

 magnetic) unit it need only move at the rate of I centim. per 

 second. If its resistance be greater it must move with a pro- 

 portionately greater velocity ; the velocity at which it must 

 move to keep up a current of unit strength being numerically 

 equal to its resistance. The resistance known as " one ohm" is 

 intended to be IO 9 absolute electromagnetic units > and therefore is 

 represented by a velocity <?/"io 9 centimetres, or ten million metres 

 (one earth-quadrant) per second. 



364. Evaluation of the Ohm. The value of the ohm in 

 absolute measure was determined by a Committee of the British 

 Association in London in 1863. It being impracticable to 

 give to a horizontal sliding - piece so high a velocity as was 

 necessitated, the velocity which corresponded to the resistance 

 of a wire was measured in the following way : A ring of wire 

 (of many turns), pivoted about a vertical axis, as in Fig. 135, 



