496 Mr Owen, On the Magnetic Deflexion of the Negative 



seconds, at the end of which time the circuit connecting the 

 insulated plate to the electrometer was broken. The deflexion of 

 the electrometer will give the current that passed from the hot 

 wire to the plate. 



The discharge was subjected to the action of a magnetic field 

 by placing the part of the tube surrounding the wire and 

 electrode between the poles of an electromagnet of the horse-shoe 

 type. The distance between the poles was about 3 cms., the 

 section of the poles was 3 cms. by 3 cms.; thus the space occupied 

 by the wire and plate was in a fairly uniform magnetic field. 

 The magnet was calibrated by observing the deflexion of a 

 ballistic galvanometer when a small coil of known effective area 

 was suddenly withdrawn from the central space between the 

 poles. The magnetic field was found to be proportional to the 

 exciting current sent through the magnet. One ampere corre- 

 sponded to 59 lines per sq. cm. Thus 15 amperes, which was the 

 largest current tried, produced a magnetic field of nearly 900 

 lines per sq. cm. As the negative discharge from a platinum wire 

 exhibits considerable irregularity, the value of the current ob- 

 served with the magnetic field on was compared with the mean 

 value of the currents without any magnetic field before and after 

 the observation. Table I. gives the results obtained on July 22nd, 

 1903, using the apparatus described above. The temperature of 

 the wire (kept constant during the set of observations) was 816° C. 

 Mean pressure during the series was 00024 mm. The potential 

 on the wire was — 120 volts. 



The last column gives the percentages of the current stopped 

 by the magnetic field. These percentages have not been calcu- 

 lated to any decimal places, as the experimental errors may easily 

 be greater than the error thus introduced. The above results are 

 shown graphically in Diagram I., where the ordinates represent 

 the percentage of the current stopped and the abscissae the 

 currents passing through the electromagnet. It will be seen that 

 the effect of the magnetic field is fairly abrupt. The value of the 

 magnetic field which begins to affect the current is somewhat 

 uncertain. The magnetic field required to stop a finite per- 

 centage (say 60 °/ ) of the current is, however, quite definite. We 

 can use this value of the magnetic field to calculate ejm for the 

 carriers of the electricity. We have 



V— 120 x 10 8 , a = 04 cms., H = 94/4 lines per sq. cm. 



Therefore e/m = 2 V/a*H 2 



2 x 120 x 10 8 



0-16 x 944 x 94-4 * 

 i.e. ejm = 17 x 10 7 . 



