Magnetic Galvanism. 85 



The remarkable want of action of magnetic galvanism upon this 

 instrument depends, I think, upon the following circumstance, which 

 I do not remember to have seen noticed by any writer upon galvanism. 

 Whenever a coil is made part of the galvanic circuit, there will be a 

 loss of power upon the galvanometer needle. I have not had time 

 to investigate whether the galvanic force is actually diminished, or 

 whether the interposition of coil, in sufficient quantity, could be made 

 to arrest the progress of chemical decomposition, but the effect upon 

 the needle is obvious, and it seems, therefore, highly probable that 

 the coil around the keeper, so necessary for the development of gal- 

 vanism, is yet the true cause of the want of power of the fluid over 

 the magnetic needle. It is obvious, from the common electro-mag- 

 netic experiments, however, that a coil has great influence in impart- 

 ing magnetism to iron, inclosed within it, and the idea occurred to 

 me that the latter could be made to represent the lost or diminished 

 galvanism of the magnet, by placing the iron close enough to the 

 needle. Trial amply confirmed my conjecture and enabled me to 

 construct a peculiarly delicate galvanometer for this variety of gal- 

 vanic fluid. The arrangement is simple. A fine wire of soft iron, 

 and equal, in length, to the magnetic needle, is to be closely bound 

 with a coil of covered copper wire, from end to end, and the extrem- 

 ities of the coil made to unite with the circuit wires from the magnet 

 and keeper. This coil of bound iron wire is to be placed at right 

 angles to the needle, and as close to it as the coils of the common 

 galvanometer. Gently sliding off the keeper always occasions a de- 

 viation, but where the motion is performed briskly, an instantaneous 

 declination of 90° and a permanent one of 80° will always follow. 

 It is obvious that the iron wire becomes polar by the passage of the 

 galvanic fluid through the investing coil, and so prompt is the effect, 

 that it is difficult to prevent the needle from describing a quarter circle. 

 Its indications are, of course, contrary to those of the common gal- 

 vanometer since the iron wire takes the opposite polarity from that 

 ' portion of the coil nearest to it. The wire may be made to receive 

 a change of poles by simply reversing the keeper or magnet, or by put- 

 ting the keeper, without any other change of position, forcibly upon 

 the magnet. The latter is almost always the least effectual mode. 



It may be regarded as objectionable to this galvanometer, that the 

 iron wire retains its polarity so long. I have observed it to hold for 

 a day, and I suspect it has a power but little short of steel, which, per- 

 haps, it derives from the close approximation of its coil. But its 



