166 McCLELLAN— USE OF OSCILLOGRAPH. [April i 4 . 



ON THE USE OF THE FALLING PLATE OSCILLO- 

 GRAPH AS A PHASE METER. 



BY WILLIAM McCLELLAN, 



RANDAL MORGAN LABORATORY OF PHYSICS, UNIVERSITY OF PENNSYLVANIA. 

 (Read April 14, /QOJ.) 



The wave form of a periodic quantity is the curve which shows 

 the magnitude of the quantity for each instant of time. It is 

 always interesting and careful examination reveals relations that 

 could hardly be discovered in any other way. In alternating cur- 

 rent calculations, however, little can be done until the wave form 

 is known accurately. There are two general methods in use, by 

 which it may be determined — the point to point method and the 

 oscillograph method. In the first, the quantity is measured by a 

 meter, through which the circuit is closed, by a revolving contact - 

 maker, for an instant at any part of the wave for which it may be 

 set. The meter then indicates the value of the quantity at that 

 particular point only. By taking such readings at various points 

 in the cycle, the whole wave may be plotted. As this process is 

 somewhat laborious, various instruments, called wave tracers, have 

 been designed to facilitate the operation. In the Rosa curve tracer 

 a double potentiometer is used. The operator fixes his eye on the 

 galvanometer, and produces balance by means of a small crank, 

 which turns the cylinder carrying the potentiometer wire. When 

 this occurs, a second lever is pulled, which automatically prints a 

 point of the curve on a paper fixed in the proper position, and also 

 turns the contact-maker to the next position. 



Either of the foregoing methods -requires considerable time to 

 plot a whole curve. The successive points are obtained from dif- 

 ferent waves. For example, a good operator can get a curve in 

 five minutes if the instrument is in order. If he is working on a 

 sixty-cycle circuit, he has obtained his curve from 18,000 succes- 

 sive waves. It will be a true curve, therefore, if he has kept his 

 conditions absolutely constant in the interval. This is always 

 troublesome to do, but particularly so in commercial work where 

 the operator seldom has control of the generator. To avoid this 



