704 Mr. A. B. Wood on the Velocities of the 



thin platinum wire was coated with thorium active deposit 

 ^activity equivalent to 0*15 m.gm. of radium) by exposing 

 overnight to a strongly emanating source of radio-thorium. 

 The active wire, slit, and photographic plate were suitably 

 mounted on separate brass blocks, which fitted rigidly into 

 a slotted brass bar graduated in millimetres (see fig. 1). 

 With this arrangement the respective distances apart of the 

 wire, slit, and plate could be easily varied and accurately 

 measured. The whole was then placed in a rectangular 

 brass light-tight box which could be quickly evacuated to a 

 pressure of the order 0*001 mm. This box was introduced 

 between the pole-pieces of a large electromagnet capable of 

 producing fields of 10,000 gauss over a pole-lace 10 cm. by 

 5 cm. with a gap of 1*35 cm. 



To obtain sharp lines and good resolution a very thin wire 

 (0*136 mm. diameter) and narrow slit (0*067 mm.) were 

 used in the experiments. The decrease of intensity of the 

 lines entailed in this way was amply compensated by their 

 increased sharpness. 



With the arrangement just outlined exposures were made 

 for (a) one hour with the field on, (b) one hour with the 

 field off, and (c) one hour with the field reversed. On 

 developing the plate after such an experiment two pairs of 

 lines symmetrically situated with regard to a single central 

 line were observed (see figs. 3 & 4, PL XVII.). 



With the object of testing the uniformity of the field a 

 series of experiments were made with varying distances 

 source-slit (Ij) and slit-plate (Z 2 ). Using the relation given 

 by Rutherford and Robinson for the deflexion (y) of the 

 ^-particles, 



TE ' ~2y ' 



we see that when l ± — 1 2 and H is constant, the deflexion (y) 

 is proportional to the square of the distance between source 

 and slit or between slit and plate, L e. 



y cc l\ 



Fig. 2 shows graphically the results obtained by using a 

 constant field (about 10,000 gauss), the distances source-slit 

 and slit-plate being equal. The two straight lines A and B 

 correspond to the two sets of a-particles of ranges 8'6 cm. 

 and 4*8 cm. respectively. It will be seen that any slight 

 deviations from the straight line have the same direction in 

 both cases, indicating that the error is not due to inaccuracy 

 of measurement of the deflexions (y). The deviations are 



