on the Electrical Resistance of Metals. 137 
arc. Each wire was one meter in length and had a resistance 
of two ohms. The slide was worked by a long handle so that 
the observer was never near the rheostat wire except for the 
few seconds occupied in reading the scale. e latter was 
directly beneath the wires and was graduated to millimeters. 
At the end of the handle a micrometer screw was so arranged 
that the slide could be moved an exact number of millimeters. 
The rheostat was connected to the resistance box by a wire of 
‘289 ohm resistance, which was taken into account in making 
all measurements. The rheostat wire was protected from the 
direct radiations of the galvanometer lamp by a wooden screen 
and no trouble from this source was experienced. 
Battery.— A. single Daniell cell of from ten to sixteen ohms_ 
resistance was used. Very rarely two cells were used, coupled 
in series. A key which could be used as a commutator, or 
simply as a make and break circuit, was introduced in the 
battery circuit. 
to a focus by means of a lens, at some little distance from the 
plate, so that the spot of light thrown upon the latter was 
about one and one-half inches in diameter. A screen, under 
the control of the observer, served to cut off the rays from the 
plate where it was desired. A cubical glass cell with a side of 
three and one-half inches and filled with a saturated solution of 
alum, was used in nearly all the experiments to cut off the 
obscure radiations of the lamp from the plate. In one series, 
sunlight was used, a beam from a heliostat placed in an adjoin- 
Ing room, being admitted through a hole in the wall. The 
spot of light on the plate was of the same dimensions as before. 
On account of the increase of temperature in the room it was 
found necessary during part of the series of experiments to 
“place the plates in a separate apartment, the light being 
admitted to them through a hole in the wall. 
Methods.—It is impossible to say @ priori, whether an effect 
of light on the electrical resistance of a metal, if there were any 
such effect, would be instantaneous or gradual. If the effect 
were an apparent one, due to some direct action of the radiation 
upon the current passing through the metal, the former wou 
probably be the case, but if the light effected some change in 
the molecular structure of the metal, it would doubtless take 
place somewhat gradually. However this may be, it was 
thought best to divide the experiments into two classes; first, © 
those to discover whether there were an instantaneous effect of 
light, and second, those to discover the effect of exposures for 
periods of from ten to fifteen minutes. iP 
