168 WRITINGS OF JOSEPH HENRY. [1840 



coil would pass through the two instruments, in an opposite 

 direction to that of the current from the other coil. The two 

 outside coils were then so adjusted, by moving them to and 

 from the middle coil, that the induced currents perfectly 

 neutralized each other in the two instruments, and the needle 

 of the galvanometer and that in the spiral were both unaf- 

 fected when the circuit of the battery was made and broken. 

 With this delicate arrangement the slightest difference in 

 the action of the two currents would be rendered perceptible ; 

 but when a zinc plate was introduced so as to screen one of 

 the coils, the needle of the galvanometer still remained per- 

 fectly stationary, indicating not the least action of the plate, 

 while the needle in the spiral became powerfully magnetic. 

 When however a plate of iron was interposed instead of the 

 one of zinc, the needle of the galvanometer was also affected. 

 52. From the foregoing results it would seem that the 

 secondary current, produced at the moment of the sudden 

 beginning or ending of a galvanic current, by making and 

 breaking contact with a cup of mercury, consists of two parts, 

 which possess different properties. One of these is of low 

 intensity, can be interrupted by a drop of water, does not mag- 

 netize hardened steel needles, and is not screened by the inter- 

 position of a plate of any metal, except iron, between the 

 conductors. The other part is of considerable intensity, is 

 not intercepted by a drop of water, develops the magnetism 

 of hardened steel, gives shocks, and is screened or neutralized 

 by a closed coil, or a plate of any kind of metal. Also, the 

 induced current produced by moving a conductor towards 

 or from a battery current, and that produced by the move- 

 ment up and down of a battery in the acid, are of the 

 nature of the first mentioned part, while the currents of the 

 third, fourth, and fifth orders partake almost exclusively of 

 the properties of the second part.* 



* [The above paper was reprinted in Silliman's American Journal of 

 Sciene, April, 1841, vol. XLi, pp. 117-152. Also, in Sturgeon's Annals of 

 Electricity, etc., vol. vii, pp. 21-56. Also, in the London and Edinburgh 

 Philosophical Magazine, June, 1841, vol. xviii, pp. 482-514.] 



