SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 253 



any method of measurement. The cycle of operation, following 

 the opening of the primary breaker contacts, consists of a rapid 

 rise of the potential applied to the spark plug from zero to a value 

 sufficient to break down the spark gap in the engine cylinder. 

 The breakdown voltage is of the order of 6000 volts and is reached 

 in a few hundred thousandths of a second. After this, a com- 

 paratively low voltage (800 volts) maintains the electric arc 

 between the spark points and lasts for a few thousandths of a 

 second. Since the interval between sparks is of the order of 

 0.05 to 0.1 second, it will be seen that the average voltage ap- 

 pUed over a complete cycle is quite low and has been found to be 

 approximately 150 volts. These pecuUar electrical conditions 

 should be kept in mind when considering the various methods of 

 measurement described below. 



The materials studied in this investigation included porcelains, 

 glass, steatite, mica, and fused silica, as these constitute the only 

 class of substances sufficiently heat resisting for use in spark 

 plugs. While the detailed studies of polarization, etc. de- 

 scribed in this report were made on only a few of the porcelain 

 samples, the same effects seemed to be present to greater or less 

 degree in all cases and the process of conduction is probably 

 similar in all of them. The work of earlier investigators^ has 

 shown the complex nature of the phenomena, but as yet no com- 

 plete and satisfactory theory has been worked out to account for 

 them. 



APPARATUS AND SPECIMENS 



Most of the work reported in this paper was done on cup-shaped 

 specimens with flat bottoms 3 mm. thick. The principal ad- 

 vantages of this type of specimen are: 



(i) The conduction takes place through the bottom of the cup, which 

 is of definite and easily measured dimensions. 



(2) The large area and small thickness of the bottom ensure a rela- 

 tively large current even with material of high conductivity. 



2 Gray, T. Phil. Mag. V, 10: 226. 1880. Haworth, H. F. Proc. Roy. 

 Soc. London 81 A: 221. 1908. Somerville, A. A. Phys. Rev. 31: 261. 1910. 

 Campbell. Nat. Phys. Lab. 11; 207. 1914. Kinnison, C. S. Proc. Amer. 

 Ceramic Soc. 17: 422. 1915. PoolE, H. H. Phil. Mag. 34: 195. 1917. Brace, 

 P. H. Trans. Amer. Electrochem. Soc. May 5, 1918. 



