218 Mr. E. Jacot on a Relation between Ionization by 



B is the cathode-ray bulb. For the production of the 

 discharge a Cox coil was used and proved very satisfactory, 

 the discharge being perfectly unidirectional at the low 

 pressures under which all the work was carried out. A 

 rectifier was at one time included in the circuit. It had the 

 effect of inconveniently reducing the density of the cathode 

 stream and proved unnecessary. The rays on passing through 

 the anticathode entered the solenoid % (shown in section) to 

 which the bulb was waxed at a. They could here be sub- 

 jected to a very uniform magnetic field of measurable 

 intensity and deflected in a magnetic spectrum *. The 

 solenoid, 13 cm. long, was wound in 3 layers, of 77 turns 

 each, on a tube of external diameter 3*70 cm. The external 

 diameter of the windings was 4*65 cm. The calculated 

 afield at the centre gave a value of H = 21 gauss per ampere. 

 On entering the solenoid, deflexion of the rays through a 

 •right-angle brought a pencil of any required velocity opposite 

 ;the orifice o, through which it emerged from the solenoid 

 and entered the ionization-chamber I. The radius of the 

 circle into which the rays are bent to enter o is equal to the 

 internal radius of the casing of the solenoid, which was 



n%ii 

 1*77 cm. From the formula p= -^ we find the velocity of 



these rays is 6'S x 10 8 x C cm. per sec, where C is the 

 current passing through the solenoid, in amperes ; from which 

 the velocity of the rays which enter the chamber corres- 

 ponding to any desired current may easily be calculated. 



After resolution into the magnetic spectrum, a small pencil 

 ot: rays of very nearly uniform velocity entered the ionization- 

 chamber through o, where, provided any external field due 

 to the solenoid had been duly balanced out, the pencil 

 travelled without further deflexion to the Faraday cylinder F. 

 This, as also the ionization-chamber and the casing of the 

 solenoid, was of brass. There was of course scattering of the 

 rays by the gas in the chamber ; but this was very small in 

 practice, and was allowed for by suitably choosing the 

 dimensions of the Faraday cylinder. The Faraday cylinder 

 was connected to a sensitive galvanometer of the Nalder type 

 {sensitiveness: 8*7 xlO -10 ampere per scale-division, at 1 

 metre). The other terminal of the galvanometer was earthed, 

 as were also the ionization-chamber and the casing of the 

 solenoid. It was the gas in I that was eventually brought 

 into contact with phosphorus. The phosphorus was contained 

 in the tube P and separated from the remainder of the 

 apparatus by wide U-tubes, Ui and U 2 , maintained at the 



* Whiddington, Proc. Roy. Soc. July 1911. 



