Frequency of Alternating Electric Currents. 161 



magnified by the raek-and-pinion D and E and the pointer F. 

 The protruded length of A is altered until the alternating 



Fig. 2. 



B 



field due to M causes the maximum resonance, and the fre- 

 quency is read off on the scale G. 



By proper choice of the dimensions of the vibrator and the 

 rack-and-pinion a very extended scale may be obtained. I 

 have constructed one instrument in which the pointer goes 

 more than twice round the circumference from 40 periods per 

 sec. to 150 periods per see, and at the middle part of the scale 

 the accuracy of reading is within 0*3 per cent. 



The electromagnet M is usually of fine wire, and has a 

 non-inductive resistance in circuit with it, so that the current 

 taken is very small. 



The reading is usually taken at the point of maximum 

 resonance. This is observed by means of the sound given out 

 or by watching the variation in amplitude either directly or 

 by mirror, light-spot, and scale. Sometimes I fix near the 

 vibrator an adjustable piece, against which it rattles or jars 

 when the resonance is sufficient. This jarring piece may be 

 made part of an electric circuit, including a lamp or an indi- 

 cating instrument, in such a way that the circuit becomes 

 broken when the greatest resonance occurs. 



The instrument can be used as a speed-indicator for 

 machinery by attaching to the rotating shaft a suitable com- 

 mutator to make and break an electric circuit carried to the 

 instrument, which may thus be at any distance. 



If we keep to simple English we might call such an 

 instrument a u Wave-teller," but to make the meaning clearer 

 I propose to name it a "Frequency-teller." 



In conclusion I may mention that since there is very little 

 inertia in the moving parts the readings can be taken rapidly, 

 and thus the instrument can be made to follow fairly quick 

 changes in frequency. 



