Measurement of Alternating Electric Currents. 159 

 and (2) becomes 



V c 



But 1° F. may be expressed as (-xj C, so that 



R (|) =R«(t), (4) 



and R/5\ will be the resistance at 1° F. 



Hence U/5\=Ua f (5) 



Thus, from (4) and (5), 



or, ^"/^j^g^. 



. ; ■ 



XIII. On Neiv Instruments for the Direct Measurement of the 

 Frequency of Alternating or Pulsating Electric Currents. 

 By Albert Campbell, B.A* 



IN many experiments with alternating currents it is a very 

 great convenience to be able to draw the supply from an 

 ordinary lighting circuit, but the value of such a source of 

 current is often lessened by the fact that the frequency of 

 alternation is usually variable, the limits of variation being 

 in some cases very wide indeed. It was for the purpose of 

 getting rid of this uncertainty that, about a year ago, I 

 devised the two instruments described below. 



Most methods of measuring frequency depend either on 

 Impedance measurements or on the production of Resonance 

 (or Synchronism). The former I avoided, for unfortunately 

 the wave-form as well as the frequency affects impedance, and 

 I aimed at an instrument which would give trustworthy 

 readings for any kind of pulsating current whatever. 



Of the Resonance (or Synchronism) methods two are 

 familiar. The first consists in running a small synchronous 

 motor and measuring its speed by a speed-counter or indicator. 

 This is a rather laborious way, and not very accurate if only 

 a tachometer is used. In the second method f a stretched 

 wire carrying the alternating current is placed in a constant 

 magnetic field ; by varying the stretching-weight the wire is 



* Communicated by the Physical Society : read May 22, 1896. 

 t Due to Professors Ayrton and Perry. 



N2 



