786 Mr. S. Rattier on the Nature of Discharge of 



they fall. There is some evidence that a-particles from 

 radioactive substances, by impinging upon a metal target, 

 may set free slow electrons from the surface of the target, 

 but the energy of a-particles is enormous compared with that 

 of: the positive ions striking the cathode in a discharge-tube. 

 It seemed important, therefore, to investigate directly the 

 role played by the positive ions in the process of the 

 discharge, and farther to carry out experiments with a view 

 to a direct study of the conditions under which positive ions 

 may, by impact with the surface of the cathode, give rise to 

 an electronic emission from the latter. 



2. The apparatus used in these experiments consists of a 

 glass tube A (fig. 1) provided with two ground joints m and 

 n, by means of which two electrodes — an aluminium plate P 

 and a strip of thin platinum foil s — may be easily introduced 

 into or removed from the apparatus. A third electrode r in 

 the shape of a rod or a small plate is introduced in the bulb 

 through the side-tube t. The platinum strip 's is coated with 

 aluminium phosphate or calcium oxide, and when electrically 

 heated by an insulated battery of accumulators provides 

 large supplies of positive or negative ions, according to 

 whether it is used a°i an anode or a cathode. The electrodes 

 may be raised to any desired potential by means of a battery 

 ot small cells or an induction-coil provided with a rectifying 

 valve and a spark-gap. By means of a Gaede pump the 

 pressure in the bulb may be reduced down to "001 mm. of 

 mercury in spite of some grease and sealing-wax being 

 always present in the apparatus, and if necessary the ex- 

 haustion is carried still further by a charcoal tube immersed 

 in liquid air. The electric current passing through the bulb, 

 varying over a wide range between 10~ 8 amp. and several 

 milliamperes, is measured either by one of two galva- 

 nometers of different sensitiveness or by a milliammeter. 



3. In the first place experiments were made in order to 

 ascertain whether the conditions of the discharge would be 

 appreciably changed by a stream of positive ions produced 

 in the vacuum-tube. It is well known that a hot cathode 

 emitting negative carriers of electricity lowers considerably 

 the sparking potential, and according to the theory men- 

 tioned above one could expect that the same effect would be 

 reached by using a hot anode emitting under an intense 

 electric force a stream of positive ions. The experiments 

 were carried out in the following way : — The heated strip 

 coated with aluminium phosphate is connected with the 

 positive terminal of the induction-coil, the cathode P being- 

 earthed and the electrode r insulated, and the pressure in 



