LINES OF MAGNETIC FORCE — THICK WIRE GALVANOMETER. 
41 
force in a given space, indicated by their concentration or separation, i. e. by their 
number in that space. Such a use of unit lines involves, I believe, no error either in 
the direction of the polarity or in the amount of force indicated at any given spot 
included in the diagrams. 
3123. The currents produced in wires, when they cross lines of magnetic force, are 
so feeble in intensity (though abundant enough in quantity, as many results show), that 
a fine wire galvanometer must of necessity offer great obstruction to their passage. 
Therefore, before entering upon further experimental inquiries, I had another galva- 
nometer constructed, in which the needles belonging to that made by Ruhmkorff 
were employed, but the coil was replaced by a single convolution of very stout wire. 
The wire was of copper, 0*2 of an inch in diameter. It passed horizontally under the 
lower needle, then, as nearly as might be, between that and the upper needle, over 
the upper, and then again between that and the lower needle. Fig. 13. 
fig. 13, and was afterwards attached to the stand, and conti- 
nued for 19 or 20 feet outside of the glass cover. Such a wire 
had abundant conducting power ; and though it passed but 
once round each needle, gave a deflection many times greater 
than that belonging to the former galvanometer. Thus when 
the ends of the 19 feet of wire were soldered together, so as to 
form one loop or circuit, the passage of the wire once between the 
poles of a horseshoe magnet (3124.), caused a deflection, or rather 
swing of the needle of above 90°. I have had a more perfect instru- 
ment, of the same kind, constructed, in which the conducting coil 
was cut out of plates of copper, so as to form a square band 0'2 of an 
inch in thickness, which passed twice round the vibration plane of 
each needle, as represented, fig. 14. The length of metal around the 
needles was 24 inches, and the galvanometer was very sensitive, but 
the experiments to be described were made chiefly with the former 
instrument. 
Fig. 14. 
3124. It was necessary, first, to ascertain the effect of certain circumstances upon 
this simple galvanometer, as to their modification of its indications. The magnet to 
be used was a compound horseshoe instrument, weighing 16 lbs., and able to support 
40 lbs. by the keeper or submagnet. It is some years since it was magnetized, and 
it is therefore, probably, in a nearly constant state as to power. The poles have the 
form delineated, fig. 1.5. Their distance apart is T375 inch, and 
the distance downwards, from their summit to the bottom or 
equator of the magnet, is 8*5 inches. The galvanometer stood 
in the prolongation of the magnetic axis, i. e. the line from 
pole to pole, and whether it were 6 or only 3 feet distant, was 
MDCCCLir. G 
Fig. 15. 
