DOVE ON THE ELECTRICITY OF INDUCTION. Ill 



and passing round 29 times at an internal distance from each 

 other of 18^ lines, formed, when in contact, the connecting wire 

 of the galvanic circuit. 



The iron cylinders and bundles of iron wires which are to be 

 tried are placed within the cyhndrical hollows of the wooden 

 screws, Mhich, electro-magnetized by the copper wire, induced 

 a current in two superposed coils composed of wire half a line 

 in thickness, wound round with silk and having each a length 

 of 400 feet. The free ends of these transversely connected induc- 

 tion-coils are joined by means of handles, the current passing 

 through the body, or by a galvanometer, and their reciprocal 

 compensation determined in both cases *. 



The equilibrium which is destroyed by introducing an iron 

 cylinder into one of the spirals is restored by gradually inserting 

 iron wires into the other spiral. In none of these experiments 

 is induction produced by the insertion of the yet unmagnetic 

 iron into the spirals which already form the connecting circuit 

 of the battery, and on that account magnetize the affected iron, 

 but by the iron already contained in the spirals becoming po- 

 larized and depolarized successively by alternately closing and 

 breaking the galvanic circuit. All the currents of which we are 

 here treating are of the kind called momentary currents. In the 

 method of observation here pursued, as has been explained above, 



* When the induction-spirals are not of the same length as the magnetizing 

 spirals upon which they are placed, and an induction-spiral indicates an in- 

 duced current of variable intensity according to the position in which it is 

 placed upon a straight electro-magnet, then an imperfectly attained compen- 

 sation may be as easily remedied by altering the position of the induction- 

 spiral towards its magnetizing spiral as by diminishing the length of wire 

 in the more powerful induction-spiral. In order now to determine which part 

 of an electro-magnet had the most powerful inducing action, a covered copper 

 wire was coiled into two spirals of 60 revolutions, which were connected by a 

 long straight end. Into each of these spirals was inserted one of the poles of 

 an electro-magnet 22 inches in length and 14 lines thick, which was surrounded 

 by a copper wire 2"''5 thick in 60 revolutions. When the compensation of the 

 spirals had been determined by the galvanometer near to the ends of the elec- 

 tro-magnet, one of the spirals was moved to a position nearer the middle, the 

 other remaining unchanged, and the connection was broken between the elec- 

 tro-magnet and the galvanic battery. Immediately great deviations were per- 

 ceptible, and moreover in a contrary direction, when the spiral which had in 

 the first instance been the more distant from the centre was made to assume 

 the nearer position. The deviations were always traceable to the spiral which 

 was most nearly approached to the centre, and they remained the same, whe- 

 ther under the direct influence of the electro-magnetized iron horseshoe or 

 whether that was made the keeper to another electro-magnet, which by closing 

 and opening the circuit was polarized and depolarized. The most advanta- 

 geous position for an induction-spiral is therefore the middle of an electro- 

 magnet. 



