402 Electro- Magnetic Experimenis. 



With a pair of plates 4 inches by 6, it Hfted 39 lbs., or more than 50 

 times its own weight. 



These experiments conclusively proved that a great developement 

 of magnetism could be effected by a very small galvanic element, and 

 also that the power of the coil was materially increased by multiply- 

 ing the number of wires, without increasing the length of each. 



The multiplication of the wires, increases the power in two ways ; 

 first, by conducting a greater quantity of galvanism, and secondly, by 

 giving it a more proper direction, for since the action of a galvanic cur- 

 rent is directly at right angles to the axis of a magnetic needle, by 

 using several shorter wires, we can wind one on each inch of the 

 length of the bar to be magnetized, so that the magnetism of each 

 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 importance 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 depend 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 

 developeing magnetism in soft iron ; and also to ascertain, if possible, 

 the most proper length of the wires to be used — 



A series of experiments were instituted jointly by Dr. Philip Ten- 

 Eyck and myself. For this purpose 1060 feet (a little more than \ of 

 a mile) of copper wire of the kind called bell wure, .045 (tooo) ^^ ^^ 

 inch in diameter, were stretched several times across the large room 

 of the Academy. 



Experiment 1. A galvanic current from a single pair of plates of 

 copper and zinc 2 inches square, was passed through the Avhole length 

 of the wire, and the effect on a galvanometer noted ; — From the 

 mean of several observations, 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. 



