SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 26 1 



applied D. C. voltage is suddenly reversed after a specimen has 

 become polarized to a considerable extent, the initial current 

 in the new direction is found to be approximately equal in mag- 

 nitude to the original current and much greater than the value 

 immediately preceding the reversal. This implies a counter 

 E. M. F. and an attempt was made to observe such an effect b}^ 

 connecting an electrostatic voltmeter across the specimen. No 

 residual deflection of this meter was observed when the supply 

 current was removed, even after long continued polarization 

 of the specimen. This result is to some extent in contradiction 

 to facts mentioned by other observers.^ 



A magneto having alternate distributor points of the same 

 polarity connected together was also used as a source of voltage 

 and the polarizing effects found to be in every way similar to those 

 obtained with a steady D. C. source of the same average voltage 

 (150 volts). 



When alternating current is applied to a fresh specimen, there 

 is no polarizing effect and the current remains constant indefi- 

 nitely, except when the current is so large as to produce heating 

 of the specimen. When alternating current is applied to a speci- 

 men which has been previously polarized by direct current, the 

 polarization disappears at a more rapid rate than if the alternating 

 current had not been appHed. 



An attempt to throw light on these complex phenomena was 

 made by applying alternating and direct current simultaneously 

 to a specimen. This was accomphshed by connecting a trans- 

 former in series with a generator. By opening the primary cir- 

 cuit of the transformer, or the field of the generator, either source 

 of K. M. F. could be ehminated without opening the circuit or 

 interfering with the current flow from the other source. The 

 A. C. voltage was measured across the transformer terminals 

 with a moving iron voltmeter, and the D. C. voltage by a d'Ar- 

 sonval type D. C. voltmeter across the generator. The alter- 

 nating current through the specimen was passed through the 

 moving coil of an electro-dynamometer, the fixed coil of which was 

 excited by an alternating current of constant magnitude and in 



« Maxwell, J. C. Electricity and Magnetism. Ed. 3. i; 393. 



