Strength and Direction of a Varying Electric Current, 101 



the action of the apparatus depends, I shall proceed to describe 

 the construction and action of the apparatus. Fig. 1 (PL II.) 

 shows the general view of the apparatus ; while figs. 2, 3, and 4 

 (PL III.) shows the details of the arrangement of the coil, mag- 

 net, &c. N and S are the poles of a powerful horseshoe magnet 

 consisting of a bundle of square bar-magnets made of very hard- 

 tempered steel. Between the poles N and S there is sus- 

 pended, by means of a fine silk thread, a coil (C), which contains 

 a great many turns of a very fine insulated wire, and whose 

 plane is at right angles to the line joining the two poles of 

 the magnet; m is a piece of soft iron fixed inside the coil, 

 nearly filling, but nowhere touching, it, and serves to intensify 

 the magnetic field in which the coil hangs. When an electric 

 current passes, the coil tends to turn round a vertical axis in 

 one direction, or in the opposite direction, according as the 

 current is positive or negative. The two weights, w, w, 

 hanging from the coil can slide up and down the inclined 

 plane (P). These weights resist the tendency of the coil to turn 

 round caused by the passage of a current through it, and serve 

 to bring the coil to its original position when the current 

 ceases. The cords by which these weights are suspended 

 pass through small holes in a piece of brass (<#), whose dis- 

 tance from the coil can be varied by moving it up and down 

 along the vertical plane (P')> an d thus the sensibility of the 

 apparatus can be altered. The strength of the field is so great 

 that the motion of the coil caused by the passage of a current 

 is almost non-oscillatory. 



Attached to the coil (C) there is a thin circular disk of 

 ebonite (D), whose axis coincides with the vertical axis about 

 which the coil is free to turn, so that any angular motion of the 

 coil causes exactly the same angular motion of the disk. This 

 disk carries, on its underside and near to a portion of its 

 circumference, a number of platinum teeth, t, t, t, &c. 

 Directly underneath these teeth, and rigidly fixed to the 

 framework of the instrument, is a vessel (V), containing 

 acidulated water, and in this vessel is provided a capillary 

 arrangement which consists of two very narrow platinum 

 plates p p (fig. 3) (which shall, hereafter, be called capillary 

 plates) , standing vertically up, side by side, from the central 

 part of the vessel, and drawing up the water of the vessel be- 

 tween them. The position of these capillary plates, when 

 everything is in its normal position, is such, that the platinum 

 tooth midway between the ends of the series t ... t is in con- 

 tact with the column of water between the capillary plates, 

 and that when the coil, and therefore the disk, is deflected to 

 the right or left, the other platinum teeth on the left or right 



