1909] on Researches in Radiotelegraphy. 679 



electromotive force, not any electromotive force but just that value 

 which corresponds to a point on the characteristic curve at which 

 there is a sudden change of curvature. 



At this point we may notice a broad generalisation which has 

 already been made by H. Brandes, viz. that any materials such as the 

 crystals mentioned, or ionised gases, which do not obey Ohm's law as 

 regards the independence of conductivity on impressed voltage, can be 

 used as radiotelegraphic receivers. It is necessary to be able to test 

 the relative sensibility of detectors to know whether any new 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 em- 

 ploying closed sending and receiving circuits which are poor radiators. 

 All the devices called wave detectors are really only oscillation detec- 

 tors, and we can therefore test their value simply by ascertaining 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 in- 

 side, 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 enclose 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 oscil- 

 lation detector to be tested either in this circuit or as a shunt across 

 the condenser. The closed receiving circuit is so constructed that 

 it may be rotated round either of three axes. 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. If the receiving circuit is rotated round 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 insensi- 

 bility 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 desiral^le 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 



Vol. XIX. (No. im) 2 y 



