392 Prof. C. Barus on the Transmission of the 



2. To turn first to the behaviour of phosphorus in contri- 

 buting in the lapse of time to the discharge of a simple air- 

 condenser whose plates are at fixed distances apart, the 

 following experiments were made. . In fig. 1, B 2 is a water- 

 battery of 48 volts, permanently charging the quadrants of 

 an Elliott electrometer, one of which is always earthed and 

 controlled by the switch S 2 . Bj is a storage-battery (20 cells 

 suffice), one pole of which is kept earthed as determined by 

 the switch Si, to be closed momentarily on charging. The 

 other terminal charges the two condensers in parallel, M, N 

 in the electrometer, and C, P for the ionization experiment. 

 The plates M and P are also permanently earthed. N com- 

 municates in the usual way with the needle of the electrometer, 

 which is thus at the same potential as the plate C. P is a 

 phosphorus grid, consisting of two sheets of wire-gauze placed 

 close together facing each other, so that between them disks 

 of phosphorus may be secured. As the air has free access to 

 P on all sides, the medium between C and P is heavily charged 

 with phosphorus " dust." The essential precautions to be 

 observed in work of this kind have been given elsewhere. 

 Barriers are placed for examination between C and P, quite 

 out of contact with the former plate. 



The arrangement of condenser selected is thus essentially 

 one in which the air at P is saturated with phosphorus 

 emanation at all times. On passing from P to this satura- 

 tion is reduced, depending on the distance apart of the plates. 

 The actual form of the condenser is shown in fig. 2, where 

 B is a hard rubber base on which the plates C, P are sup- 

 ported on metallic feet at a distance x apart. They are 

 secured by spring terminals, a, b, adjusted by clamp-screws. 

 The charging key has been drawn in diagram in fig. 1, 

 where a, b are parallel insulated metallic rods, trunnioned at 

 c. d, and there put to earth and connected with the condensers 

 respectively. The terminals of the charging circuit are e and 

 f. The levers are either top-heavy or controlled by springs 

 to the effect that contact with one side or the other is always 

 made, unless broken by special adjustment. It is frequently 

 difficult to keep these keys free from leakage, so that simple 

 devices are sometimes to be preferred. 



3. The computation of the present results of discharge may 

 be made in the usual way, The curves are obviously nearly 

 exponential. In other words, initially — dV = cYdt, dV being 

 the loss of potential in the time dt when the potential- 

 difference of the plates is V, and c being a constant. Thus 

 cV is proportional to the current flowing between the plates 

 and V = V <r~ ct . The constant c occurring in this equation has 



