172 Mr Richardson. On an attempt to detect radiation, etc. 



the diameter of the tube i and the lines of force were perpen- 

 dicular to the excited wire {i.e. they were along the dotted line H 

 of the figure). It was found necessary, to prevent the wire from 

 glowing, to put one end (a) of it to earth ; this in no way 

 interfered with the intensity of the discharge, as was shown by 

 producing the spark gap at C. 



With this apparatus experiments were made with copper, 

 brass, soft iron and aluminium wires of about "7 mm. diameter 

 and also with the steel wire 09 mm. diameter. The length of 

 exposure varied up to three hours. The other conditions were 

 changed as much as possible, e.g. the spark length was varied from 

 1 to 4 cms. and exposures were taken with and without the 

 magnetic field on. To increase the frequency of the discharges, 

 and therefore the intensity of the skin currents, the Ley den jars 

 were replaced by two small paraffin condensers of less than one 

 centimetre capacity. 



The following measurements (for the case of the thin steel 

 wire) indicate the maximum value of the current density attained 

 at the surface. The capacity of the whole system including the 

 wires was taken to be 10 cms. The self-induction was approxi- 

 mately equal to 4 x 10 4 electromagnetic units. The period of the 

 oscillations was therefore = 14 x 10 -8 seconds approximately. For 



the exponential factor e v °" ' giving the diminution of the 

 current density with the depth x from the surface, /j, was taken to 

 be = 10 3 and a = 10 4 . If A is the current density at the surface 

 that at depth x is .'. = Ae~ 6xU ) x . The voltage for a spark length 

 of 25 cms. is 75,000 or 250 electrostatic units of potential. The 

 quantity of electricity set in motion at each discharge is therefore 

 2500 electrostatic units giving an average total current through 

 the wire of 2 x 10 10 units on the same system. Hence the current 

 density at the surface of the wire during a discharge is given 

 approximately by the relation 



2ttA X "0045 I 6 -6xio3 a; ^ = 2 X 10 10 , 



J -0045 



i.e. A — 4t x 10 15 electrostatic units or 1'3 x 10 6 amperes per 

 sq. cm. 



Since the plates were unaffected in the above experiments it 

 was concluded that no photoelectric radiation was given off from 

 the wires. It was still possible that radiation might be given off 

 which had no action in a photographic plate, or whose action was 

 feeble compared with its power of ionising the air through which 

 it passed. Further experiments were therefore made to test 

 whether the air in the neighbourhood of the wire possessed con- 

 ducting properties. 



