370 GREAT MEN OF SCIENCE 



production of cathode rays of very low velocity - slow elec- 

 trons - for pure experiments in a vacuum. By subjecting 

 them to suitable electric forces, they could be accelerated or 

 retarded at will, as was already known in respect of the 

 rapidly moving electrons in discharge tubes, but they could 

 also be brought to a complete standstill and caused to re- 

 verse their direction, so that no continuous emission took 

 place. All this has become of the greatest importance in 

 applications which opened up entirely new possibilities 

 (complete rectification of alternating current, production of 

 undamped electric oscillations, and almost unlimited am- 

 plification of weak alternating currents) particularly since 

 success was obtained in producing these slow electrons in a 

 complete vacuum in a still simpler manner, and at the same 

 time in unlimited quantity, by means of their emission from 

 hot wires. It had long been known that incandescent 

 bodies allow electricity to escape; however, as long as the 

 observations were only possible in air, no results capable of 

 leading to an explanation could be obtained. 



Here also it was first of all necessary to arrange the experi- 

 ments clear of all the complication produced by the presence 

 of gas. The fact that these experiments were only possible 

 at so late a date, or at least could only be carried out with 

 certainty, was due to the continual emission of large quan- 

 tities of gas from all incandescent bodies, which results in the 

 destruction of the vacuum; only tungsten - the suitability of 

 which was not discovered until 191 3 - does not exhibit this 

 eflFect, and hence allows of perfectly definite experiments 

 being made and of satisfactory practical application, in con- 

 nection with which the experience already gained with 

 photo-electric effects was available. 



The discovery of photo-electricity by Hertz came as a 

 surprise; electrified bodies and the possibility of exposing 

 them to ultra-violet light, had already been in existence for 

 three quarters of a century, but no one had appeared to 



