178 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
through the crystal we hear sounds in the telephone due to the fact 
that the conductivity is a function of the voltage, and is therefore 
increased more by the addition than it is diminished by the substrac- 
tion of the electromotive force of the oscillations to or from the steady 
voltage of the local cell. The telephone, therefore, detects this change 
in the average value of the current by a sound emitted by it. Pro- 
fessor Pierce has discovered that several other crystals possess similar 
properties to carborundum, for example, hessite, which is a native 
crystalline telluride of silver or gold; an anatase, which is an oxide of 
titanium; and molybdenite, which is a sulphide of molybdenum. As 
regards the origin of this curious unilateral conductivity, it seems 
clear that it is not thermoelectric, but at present no entirely satisfac- 
tory theory of the action has been suggested. 
A number of forms of oscillation detector have recently been 
invented which depend on the curious fact that a slight contact 
between certain classes of conductors possesses a unilateral conduc- 
tivity, and can therefore rectify oscillations. One such detector now 
much used in Germany consists of a plumbago or graphite point, 
pressed lightly against a surface of galena. It has been found by 
Otto von Bronk that a galena-tellurium contact is even more effective. 
To the same class belongs the silicon-steel detector of Pickard. If 
such a contact is inserted across the terminals of a condenser placed in 
the receiving circuit, and if it is also in series with a telephone, the 
trains of oscillations are rectified or converted into more or less pro- 
longed gushes of electricity in one direction through the telephone. 
These coming at a frequency of several hundred per second, corre- 
sponding to the spark frequency, create a sound in the telephone, 
which can be cut up by the sending key into Morse signals. Accord- 
ing to the researches of Professor Pierce and Mr. Austin it seems clear 
in many cases that this rectifying action is not thermoelectric, since 
the rectified current is in the opposite direction to the current obtained 
by heating the junction. 
I may, then, bring to your notice some recent work on another form 
of radiotelegraphic detector, which I first described to the Royal 
Society about five years ago under the name of oscillation valve. It 
consists of an electric glow lamp, in the bulb of which is placed a 
cylinder of metal which surrounds the filament but does touch it. 
This cylinder is connected to a wire sealed through the glass. Instead 
of a cylinder, one or more metal plates are sometimes used. The 
filament may be carbon or a metallic filament, and I found some year 
or more ago that tungsten in various forms has special advantages. 
The bulb is exhausted to a high vacuum, but of course this means it 
includes highly rarefied gas of some kind. When the filament is ren- 
dered incandescent it emits electrons, and these electrons or negative 
