1884.] On a New Reflecting Galvanometer, fyc. 289 



their extremities coming into contact with the sides of the cores, 

 in the case of large deflections ; and it was intended to curve the 

 axes of the coils similarly, to allow of the use of, as short as 

 possible, a connecting-bar of aluminium. Straight cores are more 

 easily made ; but the diagram shows clearly that in this case, since it 

 is advantageous to have the cores of the coils as small as possible, 

 it is necessary to curve the needle. The needles are at present 

 arranged with their unlike poles turned in similar directions, and are 

 hung by a single fibre of unspun silk from the square screw-pin, p. 

 A nut, n, working between two fixed projecting pieces raises or lowers 

 p, and therefore the needles, without giving any twist to the thread. 



The four coils are at present arranged in series, and have a com- 

 bined resistance of 30,220 ohms made up of 5,392 yards of copper- 

 wire of No. 50 B.W.G., laid on in 62,939 turns in all, approximately 

 equally divided among the four coils. The distributing plate, 

 described below, admits of ready joining of the coils in such a way as 

 to diminish the sensibility or resistance of the instrument. When a 

 current is sent through the instrument, the poles of one horseshoe are 

 pulled farther into the corresponding coils, and the poles of the other 

 horseshoe are pushed out of the coils. A couple tending to turn the 

 system as nearly as may be round the axis of suspension is thus 

 applied ; and a mirror attached to the upright piece of aluminium, a, 

 reflects a beam of light to a scale in the usual manner, to give 9 

 means of measuring the deflection. According to one arrangement of 

 the instrument, the magnet M, carried in a horizontal position by 

 the brass rod above the needles, is used to give a return couple, 

 and to control the zero conveniently can be turned in azimuth by 

 means of the tangent-screw t. The instrument is fitted with three 

 terminals T, T T , T 2 , so that it can be used as described below as a 

 differential galvanometer. These terminals and the connecting- wires 

 are carefully insulated by vulcanite. As thus arranged we can 

 easily give this instrument a sensibility so great as to give a deflection 

 of one division (J-millim.) on a scale at a distance of rather more 

 than a metre, with one Daniell's cell and a total resistance of 

 10 11 ohms in direct circuit ; a degree of sensibility which, we think, 

 could not be reached with the most sensitive astatic instruments as 

 ordinarily constructed. With special adjustments of the needles, we 

 can obtain still greater sensibility, but the period of oscillation of the 

 needles becomes then so great as to be troublesome when results have 

 to be rapidly obtained. The instrument, however, has a very high 

 sensibility for most purposes to which it can be applied, with at the 

 same time a period of free vibration which is not inconveniently long. 

 We have found it very convenient in our experiments on the resist- 

 ance of glass at different temperatures, in the course of which, though 

 not arranged for very high sensibility, it has been used with a battery 



x 2 



