208 PROCEEDINGS OF THE AMERICAN ACADEMY 



overlooked by those who have given explanations of the phenomenon 

 based upon the ordinary laws of induction. Faraday early showed 

 that, if a copper disc be rotated between the poles of a strong electro- 

 magnet, currents of considerable strength could be drawn from it by 

 connecting one end of a wire with the axis of the disc, and the other 

 with its periphery. It has never been shown, to the writer's knowl- 

 edge, that by suddenly making and breaking the circuit of the electro- 

 magnet a degree of static induction could be produced in a copper 

 disc sufficient to produce a spark. It will be readily seen that making 

 and breaking the circuit of the electro-magnet is equivalent to cutting 

 the lines of force of the magnetic field by quick, rotation ; and therefore 

 the plienomenon possesses an interest, because it supplies a break 

 in the literature of the subject. 



NO. VI. — OX A NEW FORM OF MIRROR GALVANOMETER. 



The want of graduated circles for galvanometers is often seriously 

 felt in Physical Laboratories. The method of reading the deflections 

 of the needle by the reflection of a spot of light from a minute con- 

 cave mirror over a scale, or by the reflection of a scale in a plane 

 mirror attached to the swinging magnet, are methods of great delicacy. 



In certain cases, it is difficult to obtain suitable mirrors. I present the 

 following method of reading the deflections of a galvanometer needle, 

 without the aid of minute mirrors either plane or concave, which are 

 usually attached to the magnet. In this method, the mirror is station- 

 ary, while tiie magnet moves. I have applied the method to Helm- 

 holtz's modification of Gangain's galvanometer. In the line passing 

 through the pivot or line of suspension of the needle, not necessarily 

 above the centre of the needle, but somewhere in a line passing 

 through its centre, and perpendicular to its length, an ordinary plane 

 mirror is placed. A piece of looking-glass will answer. The silvering 

 of the upper half of the stiip of looking-glass is removed, and a fine 

 scale is etched upon it ; or for rough purposes a paper millimetre 

 scale is pasted upon the unsilvered portion. 



It will be readily seen that for small deflections -with a magnet pro- 

 vided with the ordinary long aluminmu pointers, tipped at the ends 

 with a small vertical point, if the eye be placed so that the vertical 

 point and its image in the mirror are in the same line that the projec- 

 tion of the arc of the circle of which the needle with its pointers con- 

 stitutes the diameter, can be read along the scale placed upon the 

 mirror. 



