SCIENTIFIC PROBLEMS IN RADIOTELEGRAPHY 361 



the filament to the plate but not in the opposite direction. 

 This property appears to be dependent on the fact that 

 electrons or negative ions are liberated from incandescent 

 carbon when it is at a high temperature. 



A third much used detector is the magnetic detector of 

 Marconi, in which an endless band of hard iron wire is made 

 to pass near the poles of a couple of small horseshoe magnets 

 with similar poles in contiguity, the iron being embraced at 

 that spot by two coils ; one of these coils is in connexion 

 with a telephone receiver, the other coil is free to receive 

 electric oscillations from an antenna. The oscillations shake 

 up the iron and either cause it to lose its quality of magnetic 

 hysteresis or else promote an increased permeability or power 

 of acquiring it. In either case they alter the magnetic con- 

 dition of the iron within the secondary coil and hence cause 

 an electromotive force in the latter, which in turn causes a 

 sound in the telephone. This detector is simple and easily 

 managed and is largely used in ship installations. The need 

 for a form of detector which will record the signals has called 

 forth much ingenuity. The old forms of coherer and relay and 

 Morse printer, recording in dots and dashes on paper tape, 

 are now very little used, owing to the numerous adjustments 

 required and to their sensibility to external disturbances. 

 At present, in large stations, the Marconi Company use a form 

 of Einthoren string galvanometer joined in series with a 

 crystal or glow-lamp rectifier. The deflections of the fine 

 silvered quartz fibre in the strong magnetic field are recorded 

 by photography on a prepared tape, which is developed, fixed 

 and washed as it passes through the instrument and can record 

 signals at the rate of fifty words or more a minute. 



Provided with these detectors and the associated antenna, 

 the radiotelegraphist is able to detect any and all sorts of 

 electric waves passing through space. 



The atmosphere round the earth is the seat of natural 

 electrical disturbances which give rise to vagrant electric waves, 

 called atmospheric X's or strays, which are recorded on the 

 receivers used in competition with the message-bearing waves 

 sent out from transmitting stations in correspondence with the 

 receiver. In early days, when the coherer was used in con- 

 junction with the Morse printer as receiver, these atmospherics 

 were difficult to eliminate ; they gave rise to false signals, dots 



