560 Sir E. .Rutherford and Mr. H. Robinson on Mass and 



and insulated from it, by two rings of ebonite. Using coils 

 of these dimensions, fie'ds up to 6000 gauss can be measured 

 without any overheating of the primary, and attendant varia- 

 tions in the resistance of the circuit. The current in the ' 

 primary coil was measured on a Weston ammeter, which was 

 calibrated in the usual way with a standard cell and standard 

 resistance. A Siemens and Halske ballistic galvanometer was 

 used which had a very steady zero and a period of 13 seconds. 

 The search-coils were arranged to give a deflexion of over 

 400 scale-divisions, and the current in the primary solenoid 

 was adjusted to give very nearly the same deflexion to avoid 

 possible corrections for want of proportionality of the scales. 



The distance between the centres of the two narrow bands 

 oir the photographic plate obtained by reversal of the 

 magnetic field was determined by means of the Kayser 

 measuring machine. Individual measurements of the dis- 

 tance between the bands agreed to within 1 part in 1000. 



The mean radius of curvature of the path was then deduced 

 from the formula 



where l x and l 2 have the meanings already assigned, and d is 

 the deflexion of the a. particle at the end of its path, i. e. half 

 the distance between the lines on the photographic plate 

 obtained by reversal of the field during the experiment. The 



mv . — i — 



value of -~r is then given by Rp, where H is the mean 



effective field. 



The double reflexion of the pencil of a rays was determined 

 with three separate sources in a constant effective field of 

 6236 gauss, and found to be 13*624, 13-634, 13*631 mm. 

 respectively. The average value is 13*63 mm. The values of 

 li and l 2 were 6*450 and 6*618 cm. respectively. 



The value of ~r for the a particle of radium is 



(3*983±*005)xl0 5 



(E being measured in absolute electromagnetic units), which 

 we believe to be correct to well within 1 part in 400. 



It may be recalled that the first determination of -~r for 



the rays from radium by Rutherford* gave a value of 

 3*98 x 10 5 . In a later determination the value was changed 

 to 4*06 X 10 2 . While the experiments were in progress an 

 * Kutherford, Phil. Mag. x. p. 163 (1905) ; xii. p. 348 (1906). 



