the measurement of Electrical Forces. 389 



With the help of an additional galvanometer, the resistance 

 of the helix of the one in ordinary use may be measured by 

 the above process ; and also the resistance of the secondary 

 wire of the double circuit may be estimated, by introducing 

 as the I a piece of wire having the same section and conducti- 

 bility as the secondary wire, — cut off" for instance from the 

 same piece. Thusr + w becomes known, and may then be 

 used for the determination of the resistances opposed to weaker 

 forces. If the helix of the galvanometer be skilfully formed 

 and well-insulated, the resistance r + w once determined will 

 not be liable to change. 



The sum of all the tensions of the circuit, positive and ne- 

 gative, i.e. of the affinities producing the current, is known 

 from the equation 



A=F(R + L). 



In the majority of cases A = FR will be a sufficient approxi- 

 mation; or at least A = F(R + t<y), as except in very weak 

 forces, w will be so small that it will not materially differ from 



T '■ 



L, or X w. 



r + w 



The practical form which I have given to an instrument for 

 the above purposes, and which I call the " Detached Galva- 

 nometer Scale," is represented in Plate VII., and may be thus 

 described. 



It consists essentially of four wires, o, b,c,d; the first three, 

 formed of copper, are one-fourth, one-eighth, and one-six- 

 teenth of an inch in diameter respectively ; the last, d,is also 

 one-sixteenth of an inch in diameter, but is formed of plati- 

 num. Each of them is exactly eight inches long, and they 

 are all partially sunk in grooves, parallel to, and about one 

 inch distance from, each other, in a board eight inches long of 

 hard well-dried wood, varnished and graduated into tenths of 

 an inch, the lines being drawn through from side to side. 



The four wires are joined into one continuous wire, com- 

 mencing at A and terminating at E, by being soldered to the 

 connecting pieces A, B, C, D, E, which are bored, and fur- 

 nished with screws to clasp a wire from any electromotor whose 

 force is to be measured. 



The connecting pieces are of solid brass, and at least half 

 an inch thick, in order to offer no appreciable resistance to a 

 current passing through them from the wire of the electro- 

 motor to the wire of the scale. 



When in operation, the wires from the galvanometer are 

 always to be attached to the terminating connecting pieces A 

 and E. 



