SCIENTIFIC PROBLEMS IN RADIOTELEGRAPHY 359 



wave trains are extremely violent. The reason is that each train 

 of oscillations set up in the receiving antenna by the oscillatory 

 discharge of the condenser in the distant transmitting station 

 consists of a group of electrical oscillations gradually decreasing 

 in amplitude ; the successive oscillations are repeated at intervals 

 say of a millionth part of a second, some forty or fifty oscillations 

 forming the group or train. Electrical vibrations of this fre- 

 quency cannot affect the telephone, not merely because their 

 frequency lies beyond the limits of audition but because the 

 inductance or electrical inertia of the telephone coil is too great 

 to permit sufficient current to flow through it at this frequency 

 to move the diaphragm. If, however, we connect in series with 

 the telephone some device which either rectifies these oscillations 

 or permits movement of electricity only in one direction through 

 it, the group of decrescent vibrations is changed into a pro- 

 longed gush of electricity entirely in one direction. If these 

 gushes succeed each other at the rate of several hundred a 

 second, in passing through the telephone they give rise to a 

 musical note of the same frequency as that of the spark 

 discharges in the transmitter. 



If these latter sequences of discharges are cut up into long 

 and short groups by a key, a listener at the telephone would 

 hear a series of musical sounds of long or short duration, 

 which he could interpret alphabetically on the Morse code. 

 Many such rectifiers are now known. For instance, it is a 

 property of carborundum — an artificial crystalline carbide of 

 silicon made in the electric furnace — that an electric current 

 flows more easily in one direction through the crystal than in 

 another; consequently, as the conductivity in different directions 

 is not the same, the crystal can act as a valve for electricity. 



Accordingly, a crystal of carborundum joined in series with 

 a telephone provides a means of hearing electrical oscillations 

 if broken up into groups, the group frequency being preferably 

 about 500 per second. 



G. W. Pierce has found that crystals of Hessite (a telluride 

 of silver) and Anatase (a native oxide of titanium) will act in 

 the same manner as carborundum. Again, it has been found 

 that a light contact between certain metals and non-metals is 

 a better electrical conductor in one direction than in the 

 opposite. G. W. Pickard has found that a contact between 

 steel and silicon has this property and L. W. Austin has shown 



