328 



NATURE 



[Fed. 5, 1880 



is placed on the top" of the fixed coils and insulated from 

 them. 



The deflecting coil is made of thick copper ribbon 

 fastened with insulated rivets. In its centre, and parallel 

 with it, is a light brass rod or pointer. A copper rod in 

 connection with the outer end of the coil has an iron or 

 nickel-plated point, which dips in mercury contained in a 

 double- walled metal cup, B, on the base-board. A similar 

 rod from the inner extremity of the coil ends in an iron 

 or nickel-plated cup, C, containing mercury. The coil 

 hangs under the metal cylinder, D, so that a plunger, A, 

 in the latter can dip in the mercury in the cup c of the 

 former. The suspension is of fine sewing silk, waxed or 

 treated with shellac. The thread passes over a little pulley, 

 e, above, with both parts parallel, or nearly so, and close 

 together. As represented in Fig. 1, the large coils are 

 connected in series. The current, after traversing the 

 left-hand coil, is led by a wire to the cylinder, D, thence 

 by the plunger, A, to the cup, C, through the small coil to 

 the cup, B, which is connected by a wire with the right- 

 hand coil. In order to prevent heating of the mercury 

 c ^nnections, a stream of cold water is passed through the 

 hollow plunger, A, and between the walls of B, from a jar 

 above, connected by rubber tubes. 



When the current passes, the suspended coil is power- 

 fully deflected, but its actual movement is limited by a 

 vertical wire stop, against which the pointer-rod strikes. 

 To the pointer-rod are attached, on opposite sides, two silk 

 threads which lead over pulleys on the side-bars to small 

 pans, one on each side of the instrument (Fig. 1). The 

 pulleys are light, nicely balanced, and turn on hardened 

 steel pivots. When deflection has occurred, weights are 

 added to the pan on the side opposite until the pointer- 

 rod returns to its starting-point. A scale is marked on 

 the cylinder in front of the instrument (Fig. 1), and a 

 pointer of aluminium wire is fastened to the rod in the 

 centre of the movable coil, so that it traverses the scale 

 (a more convenient mode of noting the return of the coil 

 might be taken). The pans are of the same weight, and 

 the threads by which they are hung are silk fibres. The 

 friction of the pulleys is very small and would be trifling 

 if they were made with jewelled bearings. Also one 

 balances or nearly balances the other, so that practically 

 their friction may be neglected, although allowance might 

 be made for it, if extreme nicety were aimed at. The 

 actual observation is made when the coil is in the zero 

 position, the weight taken being that required to balance 

 the deflecting force. The movement of the pulleys is then 

 very slight, and the weight acts exactly at right angles to 

 the pointer-rod. 



For the measurement of the large currents derived 

 from dynamo-electric machines, minuteness is not de- 

 manded, since the variations due to fluctuations in the 

 currents, alterations in resistance, &c, are much greater 

 than the limits of observation in such an instrument as 

 this. Quickness and simplicity of working, together with 

 strength and compactness, are required in the electro- 

 dynamometer, and this instrumentpossesses these practical 

 advantages, while it is capable of a good degree of accuracy. 



The instrument shown in the figures was made for 

 experimental trial, and is defective in certain details ; 

 still it was found to be a good working piece of appa- 

 ratus. 



Theory of the Instrument. —The expression for the 

 strength of current is very simple. The weight found is 

 that required to balance 'the deflective force and is ob- 

 served at zero, so that the earth's and local a: 

 are avoided, nor does the torsion of the suspension enter. 

 Let 



S = strength of current in webers. 



■w = weight used in milligrammes. 



/ = length of weight-arm, or distance from point where 

 weight acts, to centre of system. 



G = constant of large coils. 



g = constant of small coil. 



C = constant of instrument or length of magnetic arm. 

 By the theory of the electro-dynamometer, the force 

 acting to deflect is represented by the expression 



~ n " X i? X S-, in which -^ is the constant of the large 



r : r 



coils or G, and g the constant of deflecting coil. This 

 force acts with the arm C and is balanced by the weight 

 acting with the arm /. Hence 



r-s lw 



~~CGg 



The coils being large, G and g are readily ascertained 



from measurement, and / is a known distance. With the 

 instrument in question, C was found by passing the same 

 currents through it and through Trowbridge's dynamo- 

 meter, the constant of which was accurately known. 



C, I, G, and g being known, it is evident that from the 

 observed w, 6 may be obtained with little calculation. 

 Or, a table may be drawn up from which the value 

 desired can be derived by inspection. Also, a set of 

 weights can be prepared which will represent the current 

 in webers directly. Doubtless this will often be convenient. 



The instrument described has been worked with cur- 

 rents as small as 10 webers, but it is not quite sensitive 

 enough for such use. With those of 20 webers and 

 upwards it operates satisfactorily. Greater nicety of 



