Electro-Magnetic Apparatus. 23 

 apparatus, and particularly in those cases in which powerful mag- 

 nets cannot be applied. And such a modification appears to me to 

 be obviously pointed out in the construction of Prof. Schweigger's 

 Galvanic Multiplier :* the principles of this instrument being di- 

 rectly applicable to all the experiments in which Mr. Sturgeon's 

 improvement fails to be useful, and to those only can it be success- 

 fully applied. The following description of the figure in Plate I. 

 will render my meaning sufficiently clear. 



Fig. 1, is an apparatus on the plan of the Multiplier, to show 

 the deflection of a large magnetic needle. It consists of a coil of 

 wire, A B, of an oblong form about ten inches in length and one and 

 a half in width, with a small galvanic element attached to each end ; 

 the coil is formed of about twenty turns of fine copper or brass wire, 

 wound with silk, to prevent contact, and the whole bound together 

 so as to have the appearance of a single wire. The attachment of 

 the zinc and copper is more plainly shown in Fig. 2, which repre- 

 sents a coil of only two turns of wire: on the left side of the figure 

 the plates are soldered directly to the ends of the wire of the coil • 

 on the right, the plate of zinc Z, is attached to the part of the wire 

 ending with copper on the other side, while the plate of copper on 

 the right corresponds to the zinc on the left. By this arrange- 

 ment, we can instantly reverse the direction of the currents, and 

 deflect the needle either to the right or left, by merely holding a 

 tumbler of acidulated water so as to immerse one or the other of 

 the double plates into the fluid. The arrows at B, formed of two 

 pieces of card, are intended to show the direction of the currents 

 and they should point in the course of the wires going from the 

 copper. N S, is the needle, about nine and a half inches W 

 made by binding together several watch springs, touched sepa- 

 rately, so as to form a compound magnet ; at the end are two balls 

 of pith, to shew the movement of the needle more plainly. This 

 instrument is complete in itself, and we receive the full effect of the 

 instantaneous immersion of the galvanic element. 



Fig. 3, represents a modification of De la Rive's ring on a lar<r e 

 scale. A B, is a coil about nine inches by six, with a small cylin- 

 der of copper, enclosing another of zinc, without bottoms, sol- 

 dered to its extremities, which end at c, the whole being suspend- 

 ed by a fibre of raw silk, so as to swing freely in a cup of acidu- 

 lated water. When this apparatus is made sufficiently light, it 

 invariably places itself, after a few oscillations, at right angles 



