492 REPORT— 1867. 



ness being choseu. If thS' four quadrants are in a perfectly symmetrical 

 position round it, and if they are kept at one electric potential by a metallic 

 arc connecting the cliief electrodes outside, the needle may be strongly 

 electrified without being disturbed from its position of magnetic equilibrium ; 

 but if it is electrified, and if the external electrodes be disconnected, and any 

 diflference of potentials established between them, the needle will clearly ex- 

 perience a couple tru'ning it round its vertical axis, its two ends being driven 

 from the positive quadrants towards the negative, if it is itself positively 

 electrified. It is kept positive rather than negative in the ordinary use of 

 the instrument, because I find that when a conductor with sharp edges or 

 points is surrounded by another presenting everywhere a smooth surface, a 

 much greater difference of potentials can be established between them, with- 

 out producing disruptive discharge, if the points and edges are positive than 

 if they are negative. 



§ 10. The mirror (t) servos to indicate, bj^ reflecting a ray of light from a 

 lamp, small angular motions of the needle round the vertical axis. It is a veiy 

 light, concave, silvered glass mirror, being of only 8 millimetres Q of an inch) 

 diameter, and 22 milligrammes ( j grain) weight. I had for many years ex- 

 perienced great difficulty in getting suitable mirrors for my form of mirror 

 galvanometer; but they are now supplied in very great perfection by Mr. 

 Becker, of Messrs. EUiott Brothers, London. The focus for parallel rays is 

 about 50 centimetres (20 inches) from the mirror, and thus the rays of the 

 lamp placed at a distance of 1 metre (or 40 inches) are brought to a focus at 

 the same distance. The lamp is usually placed close behind the vertical 

 screen a little below or above the normal line of the mirror, and the image 

 is thrown on a graduated scale extending horizontally above or below the 

 apertui'e ta the screen through which the lamp sends its light. When the 

 miiTor is at its magnetic zero position the lamp is so placed that its image is, 

 as nearly as may be, in a vertical plane with itself, and not more than an 

 inch above or below its level, so that there is as little obliquity as possible 

 in the reflection, and the line traversed by the image on the screen during the 

 deflection is, as nearly as may be, straight. The distance of the lamp and 

 screen from the mirror is adjusted so as to give as perfect an image as possi- 

 ble of a fme wire which is stretched vertically in the plane of the screen 

 across the aperture through which the lamp shines on the mirror ; and with 

 Mr. Becker's miiTors I find it easy to read the horizontal motions of the dark 

 image to an accuracy of the tenth of a millimetre. In the ordinary use of 

 the instrument a white paper screen, printed from a copper-plate, is employed, 

 and the readings are commonly taken to aboiit a quarter of a scale-division ; 

 but with a little practice they may, when so much accuracy is desired, be 

 read with considerable accuracy to the tenth of a scale-division. Formerly 

 a slit in front of the lamp was used, biit the ■v^'ire giving a dark line in tho 

 middle of the image of the flame is a veiy great improvement, first intro- 

 duced by Dr. Everett in consequence of a suggestion made by Professor P. G. 

 Tait, in his experiments on the elasticity of solids made in the Natural- 

 Philosophy Laboratory of Glasgow University*. 



§ 11. The charge of the needle remains sensibly constant from hour to 

 hour, and even from day to day, in virtue of the arrangement, according to 

 Avhich it is kept in communication with sulphuaie acid in the bottom of the 



* A Driimniond light iilacecl about 70 centimetres from the mirror gives an image, on 

 a screen about 3 metres distance, brilliant enough for leclurc-illiistrations. and with suffi- 

 cient definition to allow accurate readings of tho positions on a scale marked by the image 

 of a fiiio vertical wire in front of the light. 



