(57) 



Fig. 2, 1 3 of the real size. 



two brass plates c and d unite the 

 armaments and the magnet, so that 

 they form one whole. Another pair 

 of brass plates e and /' are bent under 

 a right angle, the vertical parts are 

 soldered to c and d; the horizontal 

 parts serve to attach the apparatus to 

 the under side of the plate. Also the 

 under sides of the armaments are 

 covered by a brass plate, in which 

 a copper tube is soldered, in which the 

 iron core k fits closely (fig. 1 and 2). 

 The hollow copper cylinder can move 

 in the annular space between the outer 

 part of the iron core and the inner 

 part of the armaments a and b; in 

 the middle of the plate is a hole as large as the space between the 

 armaments, so that a small aluminium tube may be slid over the 

 lower end of the aluminium rod, to which the needles are attached. 

 The tube bears an ebonite rod, which is screwed on tlie lid of the 

 hollow cylinder. In this way the cylinder is suspended exactly in 

 the centre of the magnetic field, the currents of air being excluded 

 by the brass plates round the armaments. 



Moreover the apparatus bears a torsionhead and an arrangement 

 to move the needles in a vertical direction without their being dis- 

 charged. The length of the suspending fibre is 17 cm.; the distance 

 between the lower and the upper plate is 13.5 cm. 



As to the sensitiveness of the electrometer, it depends on the 

 suspending fibre to a great extent. With a silvered quartz fibre of 

 55 // diameter a CLARK-cell caused a deviation of 760 mm. by 

 reversal, the needles being charged at ISO volt; distance of the 

 scale 2 M. In about half a minute the position of equilibrium was 

 reached after three oscillations. 



As the total weight which the fibre had to bear, was 20 grams 

 (the hollow copper cylinder weighed 10 grams), a quartzfibre of 24 u 

 may also be used, through which the deviation would be increased 

 more than 16 times. The period, however, would be increased to 

 four times its former value. 



If this very great sensibility is not necessary, a platinum wire^) 

 annealed in a candle -flame is to be preferred because of its 



') Hallwachs. Wied. Aun. Bd. 55, p. 170, 1895. 



