390 On the measurement of Electrical Forces. 



One wire from the electromofor must be attached to A, B, C 

 or D, according to the magnitude of the force to be measured, 

 and the other wire from the electromotor must be moved along 

 in contact with the wire a, b, c or d, according as the first 

 wire is cotmected with A, B, C or D, until the galvanometer 

 needle points to 45°. 



F is a vernier which is applicable for this purpose : the 

 moveable vrire of the electromotor, being passed through the 

 small hole e and brought up again by the notch/, will lie in 

 the groove ef, and the vernier sliding on the wire of the scale 

 by the groove which is at right angles to ef, will enable the 

 operator to read off the position of the moveable wire to 

 Y^^dth of an inch. It is requisite to keep the wires of the 

 scale and vernier bright, and to rub them well with wash- 

 leather before each time of using, and the vernier will require 

 little pressure to obtain a correct indication. 



The diameter of wire c being half that of b and one-fourth 

 that of a, its section is one-fourth and one-sixteenth of these 

 respectively. Consequently, it will offer to an electric current 

 four times the resistance which is offered by an equal length 

 of b and sixteen times that offered by an equal length of a. 

 The wire d is of the same diameter as c, but being platinum 

 has a less conducting power, say one-fifth, and will therefore 

 offer five times the resistance of an equal length of c. The 

 ratio of the resistances of the wires a, b, c, d, however, since 

 different pieces of the same material are apt to differ in their 

 conducting power, and very slight differences of diameter 

 become multiplied into importance in the section, must be 

 determined by actual experiment, which is easily done with 

 the help of a constant battery; and the product of F and ttf 

 being also ascertained by experiment for one of the wires, 

 may then be known by computation for the others. As the 

 length of rf is graduated on the board into eighty divisions, 

 and indeed with the assistance of the vernier into 800, this 

 instrument is calculated to measure a scale of electric forces, 

 extending from its lowest or unity to 80 x 5 x 4 x 4 = 6400, or 

 with the vernier through 64,000°. 



With a galvanometer of moderate sensitiveness, having a 

 helix of about 1000 inches of copper wire yijjdth of an inch 

 in diameter, a force capable of decomposing water sufficient 

 to furnish (F) jg^dths of a cubic inch of hydrogen gas in one 

 minute, required {yo) six inches of wire d to be interposed 

 between its conducting wires to raise the index of the galvano- 

 meter to 45°, making the product of F and w for this wire =*36. 

 For smaller or larger electric forces, additional instruments 

 with finer or coarser wires will be requisite. 



