1895.] on the Absolute Measurement of Electrical Resistance. 607 



And if the ratio for the disc and coil is made the same as the 

 ratio for the resistance (which it is possible in practice to arrange) 

 we have 



E= M». 



If the magnetic permeability of the medium be taken as unity, M is 

 a length expressible as so many centimetres, and the number of turns 

 per second is the reciprocal of the time of one revolution of the disc. 

 Hence the resistance may be expressed as a velocity of M centimetres 

 in the time of one revolution of the disc. This makes it clear that, 

 with the assumption in regard to the permeability of the ether (or air) 

 that we have made above, any electrical resistance is expressible as a 

 velocity of so many centimetres per second. 



We can in practice make the ratio for disc and coil the same as 

 the ratio for the resistance by first sending the same current through 

 coil and resistance and then varying the speed of the disc so as to 

 make the electromotive-force between its centre and circumference 

 equal to the electromotive-force between the resistance terminals. 



How can we test the equality of the two electromotive-forces? 

 If two equal electromotive-forces act in opposite directions in the 

 same circuit the result is electrical equilibrium. We have, therefore, 

 to proceed as follows: — Place the two electromotive-forces in the 

 same circuit so as to act in opposite directions ; then if they are the 

 only electromotive-forces in the circuit, there will be an electric 

 current round the circuit due to their difference if they are unequal, 

 and no electric current at all if they are equal. The presence or 

 absence of a current may be tested by the inclusion of a galvonometer 

 in the circuit. 



In practice it is not quite so simple as this, because it is impossible 

 to make the two electromotive-forces in question the only electro- 

 motive-forces in the circuit. There will be others, and especially 

 thermo-electric forces, always present. But this difficulty may be 

 met thus : — Eeverse the two electromotive-forces we are comparing ; 

 if the current in the circuit is not changed by this reversal of both, 

 they must be equal — i. e. if the reading of the galvanometer be 

 the same, whether the balancing electromotive-forces are in one 

 direction or the other, these electromotive-forces are equal. The 

 reversal of both our electromotive-forces may be readily effected in 

 practice by simply reversing the direction of the current through the 

 coil and resistance. 



To measure our resistance in absolute measure we have then — 



1. To make the coil (SSS, Fig. 1), and the resistance (XT) 

 parts of the same main circuit (BOFSSSPXYQCB), and to 

 pass through this circuit an electric current. A commutator (C) is 

 to be inserted in the circuit in order that the direction of the current 

 may be changed at will. 



2. To insert the electromotive-force between the centre and 

 circumference of the disc (DDD) acting in one direction and the 



