294 Messrs. T. Gray and A. Gray. [Feb. 14 y 



is attached to the onter extremity of this arm, and the other to a 

 near fixed point. The distance between the fibres being small in 

 comparison with the length of the arm, small deflections would be 

 greatly multiplied. This device would, no doubt, render a greater 

 degree of skill and delicacy of manipulation necessary in the operator 

 or experimenter, but we think it or some similar plan might in some 

 cases be adopted, and the construction of these instruments renders 

 its application to them very easy. 



Fig. 3 shows a form of distributing plate which we have devised 

 for use with these instruments, or any other in which coils are joined 

 sometimes in multiple arc and sometimes in series, or partly in both 

 ways. It consists of a set of (for those instruments described in this 

 paper, which have four coils) eight brass pieces a, h, a f , b', c, d, c, df, 

 arranged as shown round a central piece jp and within an outer ring r, 

 all carried by the vulcanite base plate of the instrument. Each of 

 these brass pieces a, b, &c, can be connected with p or r, or with 

 either of the two adjacent pieces by means of plugs. The ring r is 

 divided as shown on each side of the piece s, and each of these 

 divisions can be bridged across by means of a plug when necessary ; 

 r, jp, and s are each provided with a screw terminal, which it is useful 

 to have for some applications of the plate. The connexions are as 

 follows : — Suppose the coils on one side to be distinguished by the 

 letters a, ft, and those on the other side by 7, d, and a current to be 

 flowing through all the coils so as to produce a conspiring action in 

 each. Now call the terminal of the coil a, by which the current 



