Rood — Electrical Resistance of Glass^ Quartz^ etc. 161 



Art. XX. — On the Electrical Resistance of Glass^ Quartz^ 

 Mica, Ehonite and Gutta-percha / by Ogden" N. Rood, 

 Professor of Physics in Columbia University. 



In the August number of this Journal for 1901,''^ I pub- 

 lished an account of an electrometer intended for measuring 

 very high electrical resistances, and at the same time described 

 a set of units of resistance that had been made with its help, 

 the highest of which reached to 14,000,000 of megohms. Since 

 then, with a similar set of high resistances, measurements of 

 the external and internal resistances of certain insulating sub- 

 stances have been undertaken, which may not be without 

 interest, in spite of the fact that they are to be regarded as first 

 approximations. The reason of this limitation is to be found, 

 not in any especial uncertainty in the resistance of the units 

 employed, but in another direction, viz : metallic connection 

 with substance under experiment is necessary, but mere mechan- 

 ical contact is not sufficient to abolish contact-resistance, which 

 is apt to be quite large, and various expedients were adopted 

 to remove this troublesome difficulty. The best of these 

 appeared to be the application of an amalgam of tin and mer- 

 cury, of the consistence of soft butter, to the parts in ques- 

 tion, but others were also employed; as, for example, in the 

 case of gutta-percha, it seemed fair to cement the tinfoil to it 

 with a liquid solution consisting mainly of gutta-percha, while 

 in the case of ebonite, a metallic powder was attached to its 

 surface by a very weak solution of shellac, and this again over- 

 laid by tinfoil. Another source of error lies in the time of 

 exposure to the electrical current, as the resistance of the sub- 

 stance through which it flows is apt to increase with the time. 

 The initial resistance, or more properly the average resistance 

 during the first stroke of the electrometer, was in each case 

 noted, and an effort was made to adjust matters so that this 

 should be about one minute, but very often it differed con- 

 siderably from this amount of time, a drawback which could 

 only be remedied by a large amount of extra labor and expen- 

 diture of time. It is owing then to these two factors that the 

 measurements given below are to be regarded as first approxi- 

 mations. On the other hand, no difficulty was encountered in 

 causing the current to pass through glass, quartz, mica, gutta- 

 percha and ebonite, or to flow over their surfaces. 



Mode of experimenting. — Mr. H. C. Parker determined for 

 me the resistance of five small units in the ordinary way, and 



*This Journal, vol. xii, p. 91. 



