346 Lord Rayleigh on the Measurement of Alternate Currents 



up the coil MM (see woodcut) was reduced to a single turn of 

 about 17 centim. diameter and conveyed the main current 

 (about 10 amperes) which traversed the primary circuit of 

 the transformer. This, it may be mentioned, w r as a home- 

 made instrument, somewhat of the Ruhmkorff type, and was 

 placed at a sufficient distance from the measuring apparatus. 

 The shunt-coil SS w r as of somewhat less diameter and con- 

 tained 32 convolutions. The shunt-circuit included also two 

 electric lamps, joined in series, and its terminals w r ere con- 

 nected with two points of the main circuit outside the 

 apparatus, where the difference of potentials was about 40 

 volts. Provision was made for diverting the main current at 

 pleasure from MM, and by means of a reverser the direction 

 of the current in SS could be altered, equivalent to a change 

 of e by 180°. The measurements to be made are the effects 

 of MM and of SS acting separately, and of MM and SS 

 acting together in one or both positions of the reverser. 



The best arrangement of the details of observation will 

 depend somewhat upon the particular value of e to be dealt 

 with. If this be 60°, or thereabouts, the method can be 

 applied with peculiar advantage. For by preliminary adjust- 

 ment of the coils, if movable, or by inclusion of (unknown) 

 resistance in the shunt-circuit, the deflexions due to MM and 

 SS may be made equal to one another ; so that in the case 

 supposed the same deflexion will ensue from the simultaneous 

 action of the two currents in one of the ways in which they 

 may be combined. 



This condition of things was somewhat approached in the 

 actual measures relating to the electric flame. Thus in one 

 trial the coils were adjusted so as to make the deflexions due 

 to each of the currents acting singly equal to one another. 

 The value was 40 divisions of the scale. When both currents 

 were turned on, the deflexion was 26J divisions. Thus 



A 2 =B 2 = 40, A 2 + B 2 -2ABcose=26i; 



whence 



cos 6= '67, or 6=48°. 



In a second experiment the deflexion due to both currents 

 acting together was made equal to that of the main acting 

 alone. Here 



A 2 = 40, B 2 = 71, A 2 + B 2 -2AB cos e=40; 

 whence 



cose=*665. 



The accuracy was limited by the unsteadiness of the electric 

 flame and of the primary currents (from a gas-driven De 



