424 ELECTRICAL MEASUREMENTS 



high inductances, it is advantageous to connect it to the bridge 

 through an auxiliary tuning circuit as shown in the diagram, where 

 K is a subdivided condenser and L an adjustable inductance. 

 The values of these are adjusted to produce resonance with the 

 fundamental frequency of the hummer and by this method it is 

 quite possible to obtain pressures up to 50 or 100 volts at the 

 bridge terminals if only a small amount of current is desired. 



Electrical Resonators. The most obvious method for obtain- 

 ing the current is by the use of small dynamos of high frequency, 

 but difficulties present themselves because of harmonics in the 

 wave forms. For high periodicities it is necessary to use a 

 telephone as a detector, and to obtain a good null point a pure 

 sine wave is necessary. This may be attained by eliminating, 

 by means of electrical resonators or barriers, all but the harmonic 

 of the desired frequency. It is to be kept in mind that resonating 

 arrangements partially eliminate all but a selected harmonic. 



If an efficient selective device be used, a complex wave form 

 may become advantageous, for the harmonic of the desired 

 frequency may be selected, for instance, the fundamental, the 

 third, the fifth, and so on; and one machine run at a constant 

 speed will serve for measurements at a number of different 

 frequencies. 



Fleming and Dyke Resonator. Fleming and Dyke in their 

 researches on the power factor and conductivity of dielectrics 18 

 used a small dynamo having a normal frequency of about 900 

 cycles per second. The wave form of this machine was far from 

 sinusoidal, the third and fifth harmonics being very pronounced. 

 By the use of a resonator, they were able to select and use in their 

 bridge either the fundamental of 920 periods per second, the 

 third (2,760) or the fifth harmonic (4,600), the machine being 

 operated at a constant speed. 



Referring to Fig. 249, C\ is an adjustable paraffined paper 

 condenser of 20 microfarads capacity. The adjustable induc- 

 tance LI is made by winding single layer coils of No. 14 wire on 

 two pasteboard tubes, one of which can be slipped within the 

 other. The two coils are connected in series by a flexible wire 

 and the inductance is adjusted by inserting the inner tube to a 

 greater or less extent. Three of these inductances may be neces- 

 sary to cover the range of the fundamental, the third and the 



