tHYSICS. 3?9 



lope. The leaves were S*^" long, and were projected, magnified fifteen 

 times, upon a screen divided into centimeters. The reading of the dis- 

 tance was made about twenty seconds after the first impulse ; and the 

 deflection is proportional to the square of the potential difference. The 

 condenser employed consisted of two copper plates 7*^'" in diameter, the 

 one screwed on the electroscope only being varnished. The values ob- 

 tained for Daniell cells, for bichromate cells, and for copper-zinc poten- 

 tial difference are satisfactory. (Wied. Ann., No. 7 5 Phil. Mag., August, 

 1886, V, XXII, 228.) 



Fitzgerald has devised a new galvanometer. Its peculiarities are (I) 

 the arrangement by which the coils can be measured in their place; 

 (2) the arrangement by which the circle is read with a microscope by 

 reflection mirrors attached to the magnet when the instrument is used 

 either as a sine or tangent galvanometer; and (3) an arrangement by 

 which a spot of light reads the tangents of deflection. The first ad- 

 vantage is secured by leaving the two pairs of short and long coils 

 wound in grooves, closed in outside by a glass plate, through which Ihej' 

 can be seen, and the external and internal diameter of each layer of 

 wire measured ; the transverse diameter, by looking through small holes 

 left in the ring that covers the coils outside. The reading is effected by 

 viewing a scale engraved on the inside of a horizontal ring surround- 

 ing the needle b^' reflection in two right-angled ijrisms attached to the 

 needle, which reflect opposite sides of the scale. The corresponding 

 lines in the two maps, which differ by exactly 180°, is the line at right 

 angles to the line of intersection of the reflecting planes of the prism. 

 The exact position of that line can be read by means of a micrometer 

 in the eye-piece of the microscope. The horizontal graduated ring is 

 attached through the vertical axis on which the coils, etc., turn, to the 

 base of the instrument, and so the same circle do • for reading when 

 the instrument is used as a sine galvanometer. v" means of a small 

 mirror attached to the needle at 45° to the line of susxiension a spot of 

 light can be reflected through the glass side of the instrument to a scale, 

 and then a uniform scale represents the tangents of the deflections. 

 (Nature, March, 1886, xxxiii, 455.) Carey Foster has noted the fact 

 that Bertin, in 1869, showed that the sensitiveness of the tangent galva- 

 nometer for strong currents may be increased and the usable range of 

 deflection doubled by placing the circle in a vertical plane inclined at 

 an angle of 45° to the magnetic meridian. (Nature, October, 1886, . 

 XXXIV, 546.) Gray has described a modified form of sine galvanometer, 

 in which the coil consists of a single layer of wire wound on a tube of 

 comparatively small diameter, say, ten centimeters or less, and of great 

 length, movable about a vertical axis, and carrying within it and at its 

 center the needle attached to a delicately suspended mirror. (Phil. 

 Mag., October, 1886, V, xxii, 368.) Thompson has described a modi- 

 fied form of Maxwell's galvanometer. A light frame of copper, upon 

 which is a coil of wire, is suspended between the poles of a horseshoe 



