through Exhausted Tubes without Electrodes. 331 



it seemed possible that the uniformity of the luminosity seen 

 in the discharge was due to the superposition of two stratified 

 discharges in opposite directions, the places of maximum 

 luminosity in the one fitting into those of minimum lumi- 

 nosity in the other. Since these discharges are in opposite 

 directions, they will be pushed opposite ways when a magnetic 

 force acts at right angles to them, the discharges in opposite 

 directions can thus be separated by the application of a magnetic 

 force and examined separately. In the experiment which was 

 tried with this object, a square tube was used placed outside 

 the primary, the tube at one or two places being blown out 

 into a bulb so as to allow of the wider separation of the con- 

 stituent discharges. When one of these bulbs was placed in a 

 magnetic field where the force was at right angles to the dis- 

 charge, the luminous discharge through the bulb was divided 

 into tw r o portions which were driven to opposite sides of the 

 bulb ; each of these portions was of uniform luminosity and 

 exhibited no trace of striation . It was noticed, however, in 

 making this experiment that the discharge seemed to have 

 much greater difficulty in pr?sing through the tube when 

 the electromagnet was on than when it was off. This obser- 

 vation was followed up by several other experiments, and it 

 was found that the discharge is retarded in a most remarkable 

 way by a magnetic force acting at right angles to the iine of 

 discharge. This effect is most strikingly shown when the 

 discharge passes as a ring through a spherical bulb. If such a 

 bulb is placed near a strong electromagnet, it is easy to adjust 

 the length of spark so that when the magnet is oft' a brilliant 

 discharge passes through the bulb, while when the magnet is 

 on no discharge at all can be detected. The action is very 

 striking, and the explanation of it which seems to fit in best 

 with the phenomena I have observed is that the discharge 

 through the rarefied gas does not rise to its full intensity 

 suddenly, but as it were feels its way. The gas first breaks 

 down along the line where the electromotive intensity is a 

 maximum, and a small discharge takes place along this line. 

 This discharge produces a supply of dissociated molecules 

 along which subsequent discharges can pass with greater 

 ease. Thus under the action of these electric forces the gas 

 is in a state of unstable equilibrium, since as soon as any 

 small discharge passes through it the gas becomes electrically 

 weaker and less able to resist subsequent discharges. When 

 the gas is in a magnetic field, the magnetic force acting on 

 the discharge produces a mechanical force which displaces 

 the molecules taking part in the discharge from the line of 

 maximum electric intensity, and thus subsequent discharges 



