Mr. J. N. Hearder on the Atlantic Cable, 29 



current. Mine consists of a long and delicate steelyard, which is 

 supported between two iron pillars, about 30 inches high, firmly- 

 fixed in a heavy base-board about 4' feet long and 1 foot wide. 

 From the short end of the lever hangs a steel hook for holding 

 the keeper, with a contrivance for raising or depressing it. 

 Magnets, of various kinds, can be fixed vertically on the base- 

 board, under the keeper. One of the most useful forms of 

 magnet is a U-shaped one about 13 inches high and 1| diame- 

 ter. Upon its poles is coiled a rope, consisting of 24 strands 

 of No. 16 copper wire, each 12 feet long, covered with cotton 

 and varnished previously to twisting. The form of wire rope 

 is preferred, as every strand bears the same relative position 

 with regard to its power of influencing the magnet. The ends 

 of these wires are severally connected with twenty-four pairs of 

 binding screws, fixed in a flat piece of mahogany in front of the 

 magnet, in such a manner as to admit of their being joined in 

 various modes, either collaterally or consecutively. For example, 

 they may be made to form a short conductor of 12 feet in length 

 and 24 ^A^res in thickness ; or they may be united end to end, 

 to form one continuous conductor, 288 feet in length, and a 

 single wire in thickness; or they may form any intermediate 

 length and thickness. The only diff"erence between Mr. 

 Whitehouse's magnetometer and mine, is that he has placed his 

 magnet horizontal, whereas mine is vertical. I do not mean, 

 for a moment, to imply that Mr. Whitehouse derived his ideas 

 from my instrument ; but I merely wish to state that mine 

 was made and its construction pubhshed sixteen years since, 

 whilst, I believe, Mr. Whitehouse's magnetometer is not yet 

 three years old. 



The mode in which I apply this instrument, to ascertain the 

 resistance of any conducting circuit as compared with any other, 

 is to introduce into the circuit between the voltaic battery and 

 the magnet, a known length of wire to be tested. The amount 

 of attractive force developed by it is then noted, the wire re- 

 moved, and another wire substituted for it. The attractive 

 power is again noticed, and should it vary, the length of the 

 wire then in circuit is increased or diminished until the attrac- 

 tive force is made precisely equal to that of the first experiment. 

 Their relative resistances, or, in other words, their relative con- 

 ducting powers, are thus easily determined. For instance, sup- 

 posing that the diameters of the wires are precisely equal, their 

 relative resistances will be in the inverse proportion of their 

 respective lengths, so that if a wire requires to be reduced to 

 half the length of another wire of simikir thickness to produce 

 the same eti'ect, it shows that it lias only half the conducting 

 power, or, in other words, double the resistance. 



