ON ELECTRO-DYNAMIC INDUCTION. 17 



these instances was partially intercepted by the imperfect conduction of the 

 water and the body. 



61. To exhibit the results of these experiments with still more precision, an 

 arrangement of apparatus was adopted similar to that used by Dr. Faraday, 

 and described in the fourteenth series of his researches, namely, a double gal- 

 vanometer was formed of two separate wires of equal length and thickness, and 

 wound together on the same frame; and, also, a double magnetizing spiral was 

 prepared by winding two equal wires around the same piece of hollow straw- 

 Coil No. 1, connected with the battery, was supported perpendicularly on a 

 table, and coils Nos. 3 and 4 were placed parallel to this, one on each side, to 

 receive the induction, the ends of these being so joined with those of the gal- 

 vanometer and the spiral that the induced current from the one coil would 

 pass through the two instruments, in an opposite direction to that of the cur- 

 rent 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 galvano- 

 meter and that in the spiral were both unaffected when the circuit of the bat- 

 tery was made and broken. With this delicate arrangement the slightest dif- 

 ference 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 perfectly 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 suddenly beginning or ending of a galvanic cur- 

 rent, 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 magnetize hardened steel needles, 

 and is not screened by the interposition 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 



VIII. — E 



