PEIRCE. — ELECTRICAL RESISTANCES. 395 



Ohm's law so nearly that the contact resistances were of compara- 

 tively small importauoe. The contact resistance between two pieces 

 of wood pressed together seemed to be great, since the insulation 

 resistance of a compound slab formed of two in close contact was far 

 greater than the sum of the resistances of the two taken singly. To 

 avoid any disturbing effects that might arise from injury to the sub- 

 stance at tool-cut edges, slabs were sometimes used (see the shaded 

 portion of Figure 4) much 



greater in area than the elec- X—X . 1 I. 



trodes, and this necessitated 

 the making of allowances 

 (based on experiments with 

 zinc electrodes in a tank of so- 

 lution of zinc sulphate) for 

 the effect of the spreading of 

 the lines of flow in the slab. 



This process was not entirely satisfactory, but the results are doubt- 

 less quite accurate enough for the purpose in view. Tlie mercury 

 electrodes were effectively insulated from the brass clamping bolts 

 by the intervening slabs of ebonite. 



All the slabs of stone were specially dried in the summer sun for 

 about three weeks before they were experimented on. Through the 

 kindness of Messrs. Bowker, Torrey, & Co. of Boston, I was en- 

 abled to test the specific resistances of a large number of pieces 

 of colored marble of different kinds. A vein in a piece of marble 

 used as a switch-board has been known to short-circuit a fire-alarm 

 system, and it was to be expected that the specific resistances of most 

 colored marbles would prove to be less than that of white marble. 

 In one instance, the specific resistance was as low as three megohms. 

 The single piece of sandstone which I had at my disposal had been 

 in a dry place in the laboratory for more than five years ; its specific 

 resistance was thirty megohms. The average specific resistance of 

 hard pine as given in Table II. means little. I have seen a piece of 

 this wood with a specific resistance as high as 4,000 megohms ; but 

 very resinous pieces of hard pine, however long they may have been 

 dried, seem to have low specific resistances. Some thin birch, of 

 the kind now used to separate from each other the successive plates 

 of one or two forms of storage cell, had a specific resistance, when 

 dry, of about 500 megohms. 



Jefferson Physical Laboratory, 



August, 1894. 



