RADIOTELEGRAPH Y—FLEMING. 181 
form is an improvement. It is not always possible for an inventor 
to get these tests made at real wireless telegraph stations. Moreover, 
it is no use to test over short distances, because then all detectors 
appear to be equally good. I have found, however, that we can make 
these comparative tests very easily within quite moderate distances by 
employing closed sending and receiving circuits which are poor radi- 
ators. All the devices called wave detectors are really only oscilla- 
tion detectors, and we can therefore test their value simply by ascer- 
taining how feeble an alternating current or alternating voltage they 
will detect. If we then set up in one place a square circuit of wire a 
few feet inside, and complete the circuit by a condenser and a spark 
gap, we can set up oscillations in it by means of an induction coil. I 
find that it is necessary to inclose the spark gap in a cast-iron box, 
and to blow upon the spark with a jet of air to secure silence, absence 
of emission of electromagnetic waves direct from the spark balls, and 
constancy in the oscillatory circuit. I then set up, a few score or few 
hundred feet away, a similar tuned closed oscillatory circuit, and I 
connect the oscillation detector to be tested either in this circuit or as 
a shunt across the condenser. The closed receiving circuit is so con- 
structed that it may be rotated around either of threeaxes. It is then 
generally possible to find some position of the receiving circuit such 
that no sounds are heard in a telephone connected to a highly sensi- 
tive detector associated with the circuit. This position is called the 
“zero position.” Ifthe receiving circuit is rotated around some axis, it 
begins at a certain displacement to receive signals, and the angle 
through which it has to be turned is a measure of the insensibility 
of the particular oscillation detector being used. I find, for instance, 
that it is quite easy to take one of my oscillation valves, a magnetic 
detector, an electrolytic detector, a crystal detector, or any other 
type, and arrange these in order of their sensibility by means of the 
device described. Sensibility is not, however, the only virtue which 
a wave detector should possess. It is important that it should be 
simple, easily adjusted, and not injured by the chance passage 
through it of any unusually large oscillatory currents. Another 
quality which is desirable is that it should be quantitative in its 
action, and that any change in the amplitude of the wave received 
should be accompanied by an equal change in the current which the 
detector allows to pass through the telephone. A quantitative 
oscillation detector then enables not merely signals but audible speech 
to be transmitted. In other words, it can effect wireless telephony. 
The difficulties, however, in connection with the achievement of 
wireless telephony are not so much in the receiver as in the transmit- 
ter. We have to obtain, first, the uniform production of persistent 
electromagnetic waves radiated from an antenna; and, next, we have 
to vary the amplitude of these electric waves proportionately to, and 
