645 



However, in view of the comparison between siring and telephone 

 it may be pointed out, that the maxininni sensitivity' of the latter 

 named instrument is_ by no means available in radio practice, for 

 there is a great difference between the intensity of a signal jnst 

 barely andible and one which is readable. 



It will be noticed that we only have compared the power sen- 

 sitiveness of the galvanometer and of the telephone as such, and 

 that the application of these instruments in combination with the 

 oscillating audion and with low and high fre([uency amplifiers has 

 been left out of consideration. For the sensitiveness of reception by 

 telephone in combination with the oscillating audion we may refer 

 to the paper of Austin '). He mentions that for a just audiblesignal 

 the absolute sensitiveness of the oscillating audion is 1,2 ,x 10^15 

 watts, that is to say a power, which is about 2,5 times greater 

 than that needed by the tele|)hone as such. 



For the practical use of the string galvanometer in radio-telegraphy 

 it is superfluous to try to obtain the greatest possible sensitiveness 

 of the instrument, it is not the sensitiveness which determines its 

 usefulness, since weak signals may be strengthened by means of 

 amplifying vacuumtubes without limit. The eÖiciency of a receiver 

 is much more determined by its selectivity i.e. its freedom from 

 disturbances. 



If we wish to compare the reception by the galvanometer to that 

 by the telephone from the point of view of their selectivities, we 

 must discuss once more the properties of the human ear. As is well 

 known we are able to distinguish by means of hearing many sounds 

 produced simultaneously. If we pay special attention to one of the 

 numerous musical instruments of a complete orchestra, we are able 

 to follow its performance separately. So also the Marconist can 

 distinguish the tone of a signal, although many other sounds or 

 noises of, for instance, extraneous stations or atmospheric disturb- 

 ances reach him at the same time. This secures for reception by 

 telephone an important advantage over every form of reception 

 which has the object of recording the signal graphically. In the 

 graphical image of a concert of sounds it is extremely difilicnlt to 

 follow the tone which we wish to analyse and often it will be 

 even quite impossible to do so. 



But against this disadvantage of the galvanometer there is the 



ij Louis W. Austin. Tfie measurement of radio-telegraphic signals with the 

 oscillating audion. Proceedings of the institute of Radio engineers, 1917, Vol. 5, 

 p. 239. 



