M. Righi has observed the following facts: (1) That ultra-violet rays
reduce to sensibly the same potential two metals placed near each other
(plate and gauze parallel and close); (2) That several photo-electric
couples of this kind can form a battery; (3) That a simple metallic
plate charges itself positively under the influence of radiation; (4)
That a voltaic arc formed with a zinc rod gives the strongest effect,
while the sun gives none.
[40] _Comptes Rendus_, vol. 107, p. 559.
Besides these facts he finds:--(_a_) That certain gases and vapours,
such as coal-gas and CS₂, absorb the active rays strongly.
(_b_) That if the discharging body is easily movable it recedes like an
electric windmill.
(_c_) A film of gypsum interposed between gauze and plate charges
itself negatively on the side facing the negatively charged plate.
(_d_) Radiation produces its discharging effect even on non-conductors
(ebonite and sulphur). With glass, resin and varnishes the action is
feeble, or nearly nothing.
(_e_) If the experiment is made with a copper gauze and a zinc
plate, the phenomenon nearly disappears on varnishing the gauze.
His hypothesis is that radiation produces convection of negative
electricity, the carriers being molecules of air.
(_f_) The carrying molecules move along the lines of force, and
throw electric shadows. To show this he varnishes a zinc cylinder,
all except a generating line, charges it negatively to 1,000 volts
with a dry pile, and places it parallel to a large earth-connected
plane, which has a narrow rectangular portion insulated from the
rest and communicating with an electrometer. Light only acts on the
uncovered line of the cylinder, and on turning the cylinder round
the electrometer is only deflected when it is exposed to some of
the (circular) lines of force emanating from the active line of the
cylinder.
(_g_) Radiation charges positively an insulated metal, even when it is
an enclosure with walls of the same metal; the metal being certainly
uncharged at the beginning of the experiment. The same occurs with
sulphur and ebonite. If there is a feeble initial _plus_ charge,
radiation increases it.
(_h_) While the discharging power of radiation for negative electricity
is strongest with zinc and aluminium, and slower with copper and gold,
following the Volta series; the E.M.F. set up by radiation, when it
charges things positively, is greatest with gold and carbon, and
less with zinc and aluminium; again following the Volta series, but
inversely.
(_i_) If radiation falls on an insulated metal plate connected with
an electrometer, in an enclosure of the same metal, the positive
electrification shown by the deflection of the electrometer is greater
as the plate is further from the walls of the enclosure. The action
stops when the metal has attained a certain electric density, constant
for a given metal; so the potential of a plate is naturally higher as
its capacity is less. It is thus established that radiation acts on
the particles of gas in contact with a conductor; they go away with a
negative charge, leaving _plus_ on the conductor, until an electric
density sufficient to balance this action is attained.
(_j_) It is probable that if the solar rays do not produce an effect it
is because of the absorbing action of the atmosphere. In fact, if one
places a tube whose ends are glazed with selenite between the source of
light and the metals being experimented on, the effects become sensibly
stronger when the tube is exhausted.