264 SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 



of current. A possible explanation may be developed on a basis 

 of the migration away from one electrode, of the ions carrying 

 the current, thus leaving a scarcity of carriers for the further 

 passage of current in the original direction, but providing a 

 plentiful supply for currents in the reversed direction. Another 

 suggested explanation is the formation of resisting films which 

 may cover a considerable part of the area of the electrodes but 

 which are readily removed by electrolysis on reversal of the 

 current. Tests made using platinized surfaces as electrodes 

 instead of the melted solder showed no difference of behavior. 

 It may be noted in this connection that when the samples were 

 removed after cooling, the solidified solder adhered firmly to 

 both surfaces of the cups which had been tested with direct 

 current, but could be very readily peeled off from specimens 

 which had been tested on alternating current. 



DISCUSSION OF METHODS FOR MEASURING RESISTANCES 



As a result of the data obtained in the preHminary experiments 

 just described, it was decided to adopt as the most satisfactory 

 method for the rapid comparison of different types of insulating 

 materials, the volt-ammeter method using alternating current. 

 Under these conditions the observed resistance is substantially 

 independent of the frequency, voltage and time of application, 

 and the convenient values of 60 cycles and 500 volts were adopted 

 for the later work. Figure 2 shows the typical results obtained 

 by this method and indicates the agreement attainable on suc- 

 cessive runs even at different voltages. It should be remembered, 

 however, that the results thus obtained are for the material in 

 the unpolarized state and, when in actual use in ignition systems, 

 the material may show a much higher resistance to the uni- 

 directional impulse from the magneto. 



Of other possible methods for such work, a bridge method 

 using D. C. would be objectionable because of the variable 

 amount of polarization which would occur. Attempts were 

 made to use alternating current as a source but the measurements 

 are compHcated by the effect of stray capacities shunting the 

 high resistances which are necessary, and the time required to 



