Intelligence and Miscellaneous Articles. 315 
cellent results; I do not know within what limits of pressure it 
can be employed. 
The measurement of the volumes of gas has been made by various 
methods, one of which, based on the use of electrical currents, 
was communicated to me by Professor Tait of Edinburgh; I shall 
fee it in my next communication.— Comptes Rendus, March 2, 
1885. 
ON THE ELECTROMOTIVE ACTION OF ILLUMINATED SELENIUM, 
DISCOVERED BY MR. FRITTS OF NEW YORK. BY WERNER 
SIEMENS. 
Mr. Fritts, of New York, sent me last summer a description of 
his method of preparing selenium plates sensitive to light, which 
differs from mine in many essential points, and he also sent some 
of the plates he had prepared. Unlike mine *, they do not consist 
of parallel platinum wires which are imbedded in a thin layer of 
selenium, but of a thin homogeneous layer of selenium which is 
spread on a metal plate, and after being heated, to convert the 
amorphous into crystalline selenium, is coated with fine gold-leaf. 
Mr. Fritts found that the green light which has passed through 
the gold leaf increases the electrical conductivity of the selenium 
which it traverses. In fact the conductivity of the selenium plate 
between the gold-leaf and the metal base is increased from 20 to 
200 times as much, when the sun’s rays fall vertically upon it. 
And even when illuminated by diffused light, the action is greater 
with Mr. Fritts’s preparation than with my own. One of the 
plates sent exhibited no sensitiveness to light, but had another 
and highly remarkable property, namely, that a galvanometer 
interposed between the gold-leaf and the base plate indicates the 
existence of an electrical current in the same direction as the 
motion of the light, as long as the gold-leaf is illuminated. I 
imagined at first that this current was not permanent, but resem- 
bled a polarization=current which only continued until the molecular 
modification of the selenium by the illumination was complete; and 
my first experiment appeared to support this assumption. More 
recent and more thorough experiments have convinced me that 
this supposition was erroneous. We are here dealing with a 
phenomenon which is of the greatest scientific importance. My 
experiments have shown that when the gold-leaf is illuminated, a 
difference of potential is established, which apparently is propor- 
tional to the light and which lasts as long as the illumination. 
Obscure thermal rays do not act as electromotors, and accordingly 
the assumption that the action is thermoelectric is excluded. Mr. 
Fritts assumes that the waves of light which penetrate the selenium 
are directly converted into electrical currents, and this view is 
* Monatsber. der Berl. Akad. der Wissenschaften, May 13, 1876, and 
Jun 7, 1877. 
