ACTION OF LIGHT ON SELENIUM. 
323 
Table of Resistances. 
May 8. 
May 9. 
Temperature. 
Resistance in ohms. 
Temperature. 
Resistance in ohms. 
Temperature. 
Resistance in ohms. 
7 
14° G. 
1525000 
o°c. 
938750 
15° C. 
1700000 
8 
„ 
612500 
„ 
405000 
670000 
10 
„ 
7600000 
,, 
3225000 
• „ 
5800000 
11 
„ 
14900 
„ 
16570 
16500 
14 
460000 
„ 
398000 
475000 
15 
„ 
30600 
„ 
28270 
31300 
20 
,, 
905 
,, 
1025 
1140 
21 
» 
120000 
„ 
115000 
5? 
123500 
22 
„ 
570 
,, 
472 
j, 
411 
23 
„ 
58 
„ 
63 
58 
24 
55 
53 
77 
25 
68 
” 
55 
» 
68 
Definition of Direct or Positive Current as regards the Selenium. 
When it was found that the resistance of the selenium varied with the direction of 
the current, it became very important to know what was the direction of the current in 
any particular case. We shall in future call those currents direct in which the positive 
electrode of the battery is connected with the marked electrode of the piece of selenium 
under examination. Such currents will be designated as positive or -f- currents, and 
those currents which go through the selenium in the opposite direction will be desig- 
nated as negative or — currents. 
Pesistance altered by changing the Intensity or Direction of the Current. 
The two phenomena of diminution of resistance with increased battery-power and 
change of resistance on reversing the direction of the current are closely connected and 
are both illustrated in the following experiments. 
In order to be able to reverse the current with respect to the selenium without 
affecting any other portion of the circuit, the ends of the wire electrodes of the selenium 
were made to dip into two mercury-cups fixed on to a piece of ebonite, and these cups 
were connected with the binding-screws of the Wheatstone-bridge arrangement. Thus, 
by reversing the position of the electrodes, the direction of the current in the selenium 
was reversed. The positive direction of the current was always determined by means 
of a delicately suspended magnetic needle. 
A few preliminary experiments were made in order to determine whether the change 
of resistance of the selenium with a change in the direction of the current depended in 
any way on the position of the selenium, or on the direction of the current, with respect 
to the magnetic meridian. No such connexion was found to exist. 
Mr. Willoughby Smith’s plate was then placed in a copper air bath, and its resistance 
was measured with direct and reverse currents, the temperature of the inclosure being 
observed each time. The battery-power was kept the same, being that of 10 Leclanche’s 
cells in series. 
2 z 2 
