1 82 Mr. H. Pender on the Magnetic 



of a single disk rotating in a horizontal plane, they used 

 two vertical disks rotating about horizontal axes in the same 

 line; the needle system, inclosed in a brass tube, was placed 

 between the disks, opposite their centres. Each disk was 

 surrounded by a guard-ring, both disks and gnard-rings being 

 gilded on the side facing the needle. Between the disks 

 were placed two cond en sing-plates — glass plates gilded on 

 the surfaces facing the disks. Between the condensing-plates 

 was suspended the needle. This arrangement permitted of 

 accurate calculation of the effect which should be expected. 

 The condensing-plates were charged, from a Holtz machine 

 and battery of leyden-jars, to a potential of about 5000 volts ; 

 the disks were earthed by means of metal brushes bearing 

 on studs fixed in the periphery of the disks. The speed of 

 the disks was about 1 25 revolutions per second. The deflexion 

 of the needle on reversing the electrification was from 5 to 

 8 mm. This experiment offers a means of determining the 

 ratio of the two systems of electrical units. The mean of 

 all their determinations gave for this 3*19 xlO 10 ; the deter- 

 minations which differed from this the most being 3' 74 X 10 10 

 and 2-26 xlO 10 . 



During the same winter of 1889 Himstedt* carried out 

 a series of experiments on this subject in Giessen. In his 

 first experiments he employed the same apparatus as had 

 been previously used by llontgen, with some modification in 

 details. Later, an apparatus was constructed similar to that 

 used by Rowland and Hutchinson, but considerably more 

 sensitive. Two disks of ground-glass, 20 cms. in diameter, 

 were mounted so as to revolve about horizontal axes in the 

 same line. A strip on the edge of both sides of the disks 

 was made conducting by rubbing into the ground surface a 

 thin coat of graphite. On both sides of each disk were placed 

 condensing-plates, and between the two inside plates was sus- 

 pended an astatic needle, properly shielded, the upper needle 

 being just above, the lower just below the conducting strip. 

 The disks were charged by an induction-machine and a battery 

 of leyden-jars by means of a sliding contact. With a speed 

 of 117 revolutions per second and disks charged to 5000 volts 

 (condenser-plates earthed), a deflexion of 100 mm. on a scale 

 3 m. distant was obtained on reversing the sign of electri- 

 fication of the disks. This apparatus did not permit of ready 

 calculation of the deflexion to be expected, so no absolute 

 measurements were made. Himstedt, however, showed that 

 the deflexion was in the direction to be expected, reversed 

 with the direction of rotation of the disks, was proportional 

 * Wied. Ann. xxxviii. p. 560 (1889). 



