68 Royal Society : — Rev. T. R. Robinson on increasing 



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 con- 

 stants 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. 



Layers. 



Spires. 



Entire 

 diameter. 



Height. 



Potential. ■ Resistance. 

 P. r. 



G 



H 



I 



17,070 

 17,070 

 8,110 

 8,no 

 8,130 

 8,130 

 7,000 

 9,200 



in. 



0*0092 

 0-0092 

 0-0153 

 0*0153 

 0*0192 

 0-0192 

 0*0107 

 0*0107 



73 

 73 

 55 

 55 

 29 

 29 



i3. 6 55 



i3. 6 55 

 6,570 

 6,570 

 6,524 

 6,524 

 6,189 

 8,135 



in. 

 6*84 

 6*84 



6-72 

 6-72 



6*21 



6-56 



5'93 

 5-93 



in. 

 4-0 

 4-0 

 4-0 

 4-0 

 6*o 

 5'9 

 3'5 

 4-0 



I'COOCO 



rococo 

 0*48 1 14 

 0*48114 

 0*47520 

 0*47520 

 0*44673 

 0*60272 



rococo 

 0*85270 

 0*10079 

 0-10079 

 0*08073 

 0*08282 



] 0-51808 



K 



M 



N 



A 



B 





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 fuhlhebel which read to O'OOOl : each is a 

 mean of ten measures 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 insulated 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 the primary P'" (where they are a maximum). For G I com- 

 puted them in four other positions, and had the curiosity also to 

 measure the currents. 

 Distance of Gfrom centre =0, potential 1*0000, current T000O 



1. 



2, 



4, 



All but the last agree tolerably. 



0-9842, „ 0-9856 

 0-9790, „ 0*9746 

 0-9488, „ 0-9488 

 0-8798, „ 0-9181 

 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 cell and a tangent rheometer of 950 spires, first the helix G, 

 then that to be determined, and lastly the sum of it and G. Assu- 

 ming G= 1, we have three equations to determine — the r of the helix 

 in question, the remaining r of the circuit, and the electromotive force 

 of the cell. In deducing the currents, the term involving sin 2 was 

 * Three 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 001 in. pressure, are as 



G en tire 1 *0000, interval 1 -0000 



G4-H „ 10348, „ 1-5580 



V+V „ 0-9994, „ 1-8844 



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

 with the tension. 



