390 PROCEEDINGS OF THE AMERICAN ACADEMY. 



XV. 



ON THE ELECTRICAL RESISTANCES OF CERTAIN 



POOR CONDUCTORS. 



By B. O. Peirce. 



Presented October 10, 1894. 



Since the subject of electricity began to be studied seriously, many 

 experimenters have made lists of substances arranged in the order of 

 their electrical conductivities. These lists have not agreed with 

 one another in all respects ; but at one end of every one of them 

 metals have stood, and at the other end such insulating substances 

 as ebonite, glass, paraffine, shellac, and mica. Somewhere between 

 these extremes have appeared the so called " half-conductors," * like 

 wood and some kinds of stone. How these latter are to be classed 

 depends very much, of course, upon the uses to which they are put. 

 For work with the small charges and high potentials of experiments 

 in electrostatics, we must generally consider wood as a conductor; 

 while, for practical purposes, we may regard the wooden base upon 

 which a telegraph instrument is mounted as a perfect insulator. 



In makiug electrical measurements in the laboratory, it is often 

 necessary to be able to change quickly the connections of one's 

 apparatus, and for this purpose some kind of " switch-board " must 

 generally be provided. Sometimes a dry wooden board, into which holes 

 have been bored to form mercury cups, will suffice ; sometimes a plate 

 of ebonite or a non-combustible slab of slate or marble is required. 



I have been obliged, during the last three years, to procure several 

 hundred more or less complicated switch-boards, and many of these 

 had to be used in making accurate measurements of electrical quanti- 

 ties. It has been necessary, therefore, to determine under what cir- 

 cumstances hard dry wood or red vulcanized filjre may safely be 

 used, and when marble or ebonite, or even a block of freshly scraped 

 paraffine, is required. For use with these switch-boards I have pro- 

 vided several hundred resistance coils of German silver, platinoid, 



* Du Moncel, Annates de Chimie et de Pliysique, [5.], X. 1877 ; Addenbrooke, 

 Muir and Jamieson's Pocket-Book, p. 194. 



