THE VACUUM THERE 's SOMETHING IN IT WHITNEY 205 



previous lower temperature the load or lighting current for the lamp 

 is carried as before by electrons emitted from the thorium sur- 

 face. This is a proof of the production of a layer of thorium on the 

 tungsten only one atom deep. The electron current from thor- 

 ium on tungsten is greater than from pure tungsten, and also than 

 pure or massive thorium, and is maximum when the single atom 

 layer is present. This is confirmed by experiments on partial recov- 

 ery of the surface and supported by thousands of successive repeti- 

 tions of this experiment on one filament. 



HIGH FREQUENCY 



By means^ of an apparatus such as shown in Plate 5, Figure 1, an 

 ordinary incandescent lamp may be lighted by being brought within 

 a foot of a coil which is carrying a current of several million cycles. 

 This, produced by pulsations, is about as near to wireless transmis- 

 sion of power as anything we now have. This high-frequency prin- 

 ciple is also being used by Professor Northrup, of Princeton, for 

 special electric furnaces. In these, the induced currents in the ma- 

 terial of the crucibles or the material to be heated generate high 

 temperature through local resistance. 



LOUD SPEAKER 



Plate 5, Figure 2, shows a loud speaker of unusual design. It 

 consists of a 26-inch flat conducting disc, in a magnetic field, the vi- 

 brations of which correspond to the voice currents and reproduce the 

 sound waves without the intervention of a horn. To operate this 

 device, vacuum tubes are made use of as follows: 



Sound waves entering a microphone cause feeble electromotive 

 forces to be generated in the microphone. These feeble electromotive 

 forces are applied to the grid of a pliotron and cause relatively large 

 variations in the electric current flowing between filament and plate, 

 which in turn are used to secure larger electromotive forces to be 

 applied to the grid of another pliotron. By the use of several am- 

 plifying pliotrons the original feeble electric currents are multiplied 

 several thousand times and supplied to the loud speaker which repro- 

 duces the original sounds with many times the original volume and 

 great faithfulness of quality. To operate this pliotron amplifier re- 

 quires a direct current of several hundred volts. This may be ob- 

 tained by first transforming power from the ordinary alternating- 

 current lighting circuit to a relatively high voltage, next rectifying 

 this high-voltage alternating current by means of kenotrons, and 

 finally smoothing out this pulsating current by means of appropriate 

 electric circuits. 



