Prof. Thomson, Do the 7 rays carry a charge, etc. 121 



Do the 7 rays carry a charge of negative electricity? By 

 J. J. Thomson, M.A., F.R.S., Cavendish Professor of Experimental 

 Physics. 



[Bead 30 January 1905.] 



As the result of a series of interesting experiments Paschen 

 (PhysikaliscJie Zeitschrift, V. p. 563) came to the conclusion that 

 the 7 rays emitted by radium are, like the /3 rays, charged with 

 negative electricity. Paschen's experiments are of two types ; in 

 the first type the radium is placed inside an insulated box made of 

 thick lead, the lead being thick enough to absorb all the ft rays, 

 but not so thick as to stop more than a fraction of the 7 rays. 

 Under these circumstances the insulated box gets charged up with 

 positive electricity; this Paschen regards as proving that the 7 rays 

 which have passed through the lead have carried with them 

 charges of negative electricity, leaving an excess of positive behind. 

 The positive charge on the box may, however, be explained in 

 another way ; we know that when 7 rays enter or leave a piece of 

 metal secondary rays are produced which like the /3 rays carry 

 with them a charge of negative electricity, thus the negative 

 electricity which escapes from the box may be carried by the 

 secondary and not by the 7 rays. The experiment in this form is 

 inconclusive : to make it conclusive it would be necessary to place 

 the lead box in a magnetic field strong enough to prevent the 

 negatively electrified particles from leaving the box ; if the 7 rays 

 carry the charge the magnetic field would not diminish the 

 positive charge on the box, as they are not affected by magnetic 

 force ; if the secondary rays carry the charge the magnetic field 

 would stop the charging up of the box. 



In the second type of Paschen's experiments an insulated piece 

 of metal was bombarded by 7 rays, and the metal was found to 

 acquire a charge of negative electricity. Here again the result 

 seems inconclusive, for to prevent the charge leaking from a 

 charged body exposed to radium the body must be placed in a good 

 vacuum : thus the metal in Paschen's experiments had to be placed 

 in a vessel from which the air could be exhausted; now the Avails 

 of the vessel are struck by the 7 rays and emit secondary radiation 

 which carries a charge of negative electricity, thus the negative 

 electrification may again be due to secondary radiation and not 

 directly to the 7 rays. 



