608 



Mr. J. Viriamu Jones 



[May 24, 



electromotive-force between the extremities of the resistance acting in 

 the opposite direction in a second circuit (X M G Y X) containing a 

 sufficiently sensitive galvanometer G. Brush contacts must be made 

 at and M. 



3. To vary the rate of rotation of the disc until the reading of 

 the galvanometer is the same whether the current in the main circuit 

 through coil and resistance is being sent by the commutator in one 

 direction or the other. 



4. To measure this equilibrium rate of rotation. 



5. To multiply this equilibrium rate of rotation (number of 

 turns per second) by the coefficient of mutual induction of the coil 

 and disc, which is calculated once for all from their measured dimen- 

 sions. This product gives the resistance to be measured in absolute 

 units. 



Fig. 1. 



[The necessary arrangements were here experimentally shown by 

 the lecturer.] 



I trust that I have said sufficient to make clear to you the general 

 theory of the measurement of a resistance in absolute measure by this 

 method. It now remains to consider very briefly the practical side 

 of the matter, the difficulties that arise, the way in which they may 

 be met, and the accuracy attainable. The time at my disposal does 

 not allow me to do this at all completely, and I must content myself 

 with touching on a few points of special importance and interest. 



The first great desideratum is that we should have the rate of 

 rotation of the disc well under control, that it should run as uniformly 

 as possible, and that its rate should be capable of sufficiently exact 

 determination. 



The disc, axle, and bearings in my apparatus at Cardiff are of 

 phosphor bronze (Figs. 2 and 3). The disc is insulated from the 



