40 WRITINGS OF JOSEPH HENRY. [1831 



inch will be developed, by a separate wire ; in this way the 

 action of each particular coil becomes very nearly at right 

 angles to the axis of the bar, and consequently, the effect is 

 the greatest possible. This principle is of much greater im- 

 portance when large bars are used. The advantage of a 

 greater conducting power from using several wires might in 

 a less degree be obtained by substituting for them one large 

 wire of equal sectional area, but in this case the obliquity of 

 the spiral would be much greater and consequently the 

 magnetic action less ; besides this, the effect appears to de- 

 pend in some degree on the number of turns which is much 

 increased by using a number of small wires.* 



In order to determine to what extent the coil could be 

 applied in developing magnetism in soft iron; and also to 

 ascertain, if possible, the most proper length of the wires to 

 be used — 



A series of experiments was instituted joinly by Dr. 

 Philip Ten Eyck and myself. For this purpose 1060 feet 

 (a little more than 3^ of a mile) of copper wire of the kind 

 called bell-wire, .045 (y^lu) of an inch in diameter, were 

 stretched several times across the large room of the Acad- 

 emy. 



Experiment 1. A galvanic current from a single pair of 

 plates of copper and zinc two inches square, was passed 

 through the whole length of the wire, and the effect on a 

 galvanometer noted; — From the mean of several observa- 

 tions, the deflection of the needle was 15°. 



Exp. 2. A current from the same plates was passed through 

 half the above length (or 530 feet) of wire, the deflection in 

 this instance was 21°. 



By a reference to a Trigonometrical table, it will be seen 

 that the natural tangents of 15° and 21° are very nearly in 

 the ratio of the square roots of 1 and 2, or of the relative 

 lengths of the wires in these two experiments. 



* Several small wires conduct more common electricity from the machine 

 than one large wire of equal sectional area ; the same is probably the case 

 though in a less degree, in galvanism. 



