42 



ELECTRO-MAGNETISM. ' 



previously been untwisted, so that the 

 needle is again restored to its former 

 position. The quantities of electricity 

 circulating round the wires will be, di- 

 rectly proportional to the number of 

 degrees through which the thread has 

 been twisted. 



(118.) In the common galvanometers, 

 in which the force of the current is esti- 

 mated by the degrees of the deviation 

 of the needle, this deflecting force acts 

 with mechanical disadvantage as the 

 needle deviates from the coil. When it 

 has been deflected nearly ninety degrees 

 from its original position, an addition to 

 the power will produce scarcely any 

 addition to the effect ; and consequently 

 the instrument ceases to give indication 

 of a more energetic current. Hence 

 Mr. Ritchie's instrument is better enti- 

 tled to the appellation of a galvanometer, 

 or measurer of galvanic electricity, than 

 the former, which are mere galvano- 

 scopes, or indicators of the presence of 

 a galvanic current. It has, however, 

 the disadvantage of not being so sensi- 

 ble to the influence of feeble voltaic 

 electricity ; since the needle, being on 

 the outside of the coil, is acted upon 

 only by the difference of the two con- 

 trary electro-magnetic forces, arising 

 from the opposite currents in the upper 

 and lower parts of the coil. In the 

 former arrangement, the needle, being 

 between these two parts of the coil, is 

 deflected by the sum of these forces. 



(119.) The sensibility of the galva- 

 noscope may be very much increased by 

 neutralizing the directive force of the 

 needle arising from the magnetic influ- 

 ence of the earth. Professor Gum- 

 ming employed for that purpose a mag- 

 netized needle placed immediately be- 

 neath the moveable needle *. Nobili 

 improved upon this idea by attaching 

 the neutralizing needle to the principal 

 one, placing them one above the other, 

 and parallel to each other, but with 

 their poles in opposite directions. They 

 are fixed by being passed through a 

 straw, suspended from a thread, as in 

 the apparatus formerly described. The 

 distance between the needles is such as 

 to allow of the upper coil of the wires 

 to pass between them, an opening being 

 purposely left, by the separation of the 

 wires at the middle of that coil, for 

 allowing the middle of the straw to pass 

 freely through it. A graduated circle, 

 on which the deviation of the needle is 



* Transactions of the Cambridge Philosophical 

 Society, vol. i. p. 279. 



measured, is placed over the wire on the 

 upper surface of the frame of the in- 

 strument, having an aperture in its cen- 

 tre for the free passage of the needle 

 and straw. The whole of this arrange- 

 ment may be understood by a reference 

 to/g-. 80, which represents a section of 

 the apparatus : s n is the lower needle 



Fig. 80. 



surrounded by the coil of wire, and con- 

 nected with the upper needle N S, by 

 the intermediate straw shaft which is 

 seen to pass through the upper horizon- 

 tal coil of the wires, and also through 

 the central aperture of the card imme- 

 diately above it, on which the graduated 

 circle is drawn. In Nobili's instrument, 

 the frame was twenty-two lines long, 

 twelve wide, and six high. The wire 

 was of copper, covered with silk, one- 

 fifth of a line in diameter, and from 

 twenty- nine to thirty feet in length ; 

 making seventy-two revolutions round 

 the frame. The needles were twenty- 

 two lines long, three lines wide, a quar- 

 ter of a line thick, and they were placed 

 on the straw five lines apart from each 

 other. 



(120.) The adjustment of the opposing 

 polarities of the two needles should be 

 such, that the directive power of the com- 

 bination resulting from the magnetism 

 of the earth is very nearly balanced ; 

 the compound needle being allowed to 

 retain only sufficient power to bring 

 it to a constant position when unin- 

 fluenced by any electrical current. But 

 the peculiar excellence of the contrivance 



