500 Intelligence and Miscellaneous Articles. 



is the means applied for damping the oscillations of the combined 

 system of five needles. This consists of a thin copper cylinder, 

 movable in a strong magnetic field, and attached coaxially to the 

 aluminium stem carrying the needles, which is continued down- 

 wards below the quadrants for this purpose. The magnetic field 

 in which the cylinder moves is formed as follows : — a permanent 

 magnet, whose axis forms a nearly complete horizontal circle, is 

 fastened close below the sole-plate of the instrument, so that the 

 axis of suspension of the needles and copper cylinder passes 

 through the centre of the space between the poles, which are 

 furnished with soft iron pole-pieces in the opposing faces of 

 which grooves of semicircular section are cut. A cylindrical 

 space is thus formed, which is closed on all sides ; above, by the 

 sole-plate of the electrometer (except for a hole large enough to 

 let the aluminium stem carrying the copper cylinder pass freely) ; 

 below, by a brass plate carrying a short copper tube in which an 

 iron core of rather smaller radius than the copper damping cylinder 

 fits closely and can be slid up or down with slight friction ; at the 

 sides by brass plates closing the air-gaps between the pole-pieces. 

 The damping cylinder is thus thoroughly protected from air-cur- 

 rents and moves in the annular space between the hollowed out 

 surfaces of the pole-pieces and the iron core just mentioned. 



In order to be able to regulate the clamping, the copper cylinder 

 is adjusted, by thinning and shortening, until the movement is 

 just aperiodic when the iron core is pushed right up. Then by 

 moving the core downward, any required degree of damping can 

 be obtained. The total suspended system weighs 20 grammes. 

 The suspending fibre is 17 cm. long and is attached at the top to 

 a torsion head. The author has tried a silvered quartz fibre of 

 55 fi diameter, but finds an annealed platinum wire of 50 jjl on the 

 whole the most convenient. AVith this suspension the half-period 

 was about 12 seconds, and, as the damping was so regulated that 

 the needles came to rest after three swings, readings could be 

 taken at intervals of about 36 seconds. The needles were charged 

 by being connected with the positive terminal of a set of 300 

 Leclanche cells, the negative of which was to earth. With a scale- 

 distance of 2 metres, a Clark cell gave, by reversal, a deviation of 

 about 600 mm., and the same values could be obtained repeatedly 

 within 2 or 3 tenths of a millimetre ; in other words, measurements 

 can be made with the electrometer with an accuracy of fully one 

 part in 1000. 



The author gives a series of readings illustrating the proportion- 

 ality between potential-difference and deflection, and also examples 

 of the use of the instrument for the comparison of electromotive 

 forces, of current-strengths by the potentiometer method, and of 

 the constant of a galvanometer. — (Abstract of paper communicated 

 by the Author from the Proceedings of the lioyal Academy of Sciences, 

 Amsterdam, for June 25th, 1898.) 



