252 SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 



This very distinct species was described by Urban from Shafer's 

 nos. 117 1 and 1549, of which the former is here selected as type. Ur- 

 ban gives the maximum size of the leaflets as 2.5 cm. long, and 1.7 cm. 

 wide, and the fruit as having wings up to 2 mm. broad. 



Although the fruit of this species is very different in appearance from 

 that of the other members of the genus, owing to the great reduction 

 of the wings, the difference is only a comparative one. In its floral 

 structure I. cuhensis agrees precisely with other members of the genus, 

 and the morphology of the fruit is the same. 



ELECTRICITY. — Methods oj 'measuring conductivity of in- 

 sulating materials at high temperatures. F. B. Sii,SBEE and 

 R. K. HoNAMAN, Bureau of Standards. 



The piu-pose of this paper is to describe some measurements 

 carried out at the Bureau of Standards during the past two years, 

 on the resistance of various insulating materials at high tempera- 

 tures. This work was undertaken with a view to studying the 

 relative merits of various insulators for use in spark plugs, and in 

 particular to assist the Ceramic Laboratory of the Bureau in 

 developing improved porcelain bodies for this purpose. The 

 method finally adopted, as a result of this work, for the com- 

 parative testing of materials is described elsewhere.^ The 

 present paper will be confined to a description of the various 

 phenomena observed in the experiments which led to the method 

 finally adopted. 



The electrical and thermal conditions under which a spark 

 plug is required to operate differ considerably with the type of 

 gasoline engine used. Measurements with imbedded thermo- 

 couples have shown that the temperature of the body of the in- 

 sulator within the metal shell seldom exceeds 250° C. in water- 

 cooled engines. The tip of the inner end, however, may reach 

 temperatures as high as 900 to 1000° C. It therefore appeared 

 desirable to study the resistivity of the specimens in the range of 

 temperature between 200 and 900° C. 



The electrical stresses applied to a spark plug insulator by the 

 average magneto or battery coil ignition system used for firing 

 gasoline engines, are quite pecuHar and difficult to dupUcate in 



* Report of the National Advisory Committee for Aeronautics, 191 8. 



