176 



Rev. T. R. Robinson on increasing the 



[May 31, 



five sections. This, though so potent in respect of sparks, does not affect 

 the quantity, which with it I found as I'OOOO. without it 0*9948, a differ- 

 ence not worth noting. The case, however, would be different if there 

 were a gaseous interval in the circuit*. Beside this shelf is a bracket which 

 supports rheotomes of various kinds. 



Over the jars can be put any of the secondary helices, the constants of 

 which are given in the following Table : — 



The potential P and resistance of the first one, which I take as a standard, 

 are assumed — 1 . 



Table I. 



Name. 



Feet of 

 wire. 



Diameter 

 of wire. 



Layer. 



Spires. 



Entire 

 diameter. 



Height. 



Potential. 

 P. 



Resistance. 

 r. 







in. 







in. 



in. 









17,070 



0*0092 



73 



13.655 



6*84 



4-0 



I'OOOOO 



I'OOOOO 



H , ... 



17,070 



0*0092 



73 



13.655 



6-84 



4*0 



i-ooooo 



0-85270 



I 



8,110 



0-0I53 



55 



6.570 



672 



4-0 



0*48114 



0*10079 



K 



8,110 



0*0153 



55 



6,570 



672 



4-0 



0*48114 



0*10079 



M 



8,130 



0*0192 



29 



6,524 



6*21 



6*o 



0*47520 



0*08073 



N 



8,130 



0"OI92 



29 



6,524 



6-56 



5*9 



0*47520 



0*08282 



A......... 



7,000 



o'oio7 





6,189 



5'93 



3'5 



0*44673 



j 0*51808 



B 



9,200 



0*0107 





8,135 



5'93 



4-0 



0*60272 





The first six were made by Mr. Ladd, who also determined for me the 

 length and number of layers. The thickness of the wires was measured 

 by me with a fiihlhebel which read to 0*0001 : each is a mean of ten mea- 

 sures at different places, for no wire that I have ever tried is quite uniform. 

 The two last are experimental, their wire not being lapped, but merely in- 

 sulated by a varnish of wax and rosin, as proposed by the late Dr. Callan : 

 this plan does well for quantity, but cannot be trusted for any high tension. 



The potentials were computed, supposing the helices at the middle of 

 tke primary P'" (where they are a maximum). For G I computed them 

 in four other positions, and had the curiosity also to measure the currents. 

 Distance of G from centre =0, potential 1*0000, current 1*0000 



1, „ 0*9842, „ 0*9856 



2, „ 0*9790, „ 0-9746 



3, „ 0*9488, „ 0*9488 



4, „ 0*8798, „ 0*9181 



All but the last agree tolerably. For positions of M and N, which were 

 not central, they were specially computed. 

 The resistances were obtained by including in the circuit of a small Grove's 



* Tlu-ee of the combinations described in Table II. have tensions nearly as 1, 2, and 



3. Their quantities, with a circuit entirely metallic, and with one in which there was 



an interval of ^ inch of air at 0*01 in. pressiu'e, are as 



G entire 1*0000, interval 1*0000 



O+H „ 1*0348, „ 1*5580 



V-j-V „ 0-9994, „ 1*8844 



The first set are nearly equal ; the second increase, though at a decreasing rate, with 



the tension. 



