510 Mr. E. C. Rimington on Luminous 



Experiments 6, 7 , and 8 seem to show that ring, or other 

 shaped, sharp luminous discharges can only be obtained with 

 the wire so wound as to give magnetic induction through the 

 bulb, while the first five experiments show that an electro- 

 static field in the bulb may help the effect. The theory the 

 author has come to after consideration of the above and 

 other experiments is : — That if the E.M.F. due to rate of 

 change of magnetic induction acting in the dielectric of 

 rarefied gas be insufficient to break it down and produce a 

 luminous discharge (owing to the spark-gap being too short), 

 the electrostatic field between the plates C and D, or between 

 one of the plates and part of the wire, if correctly timed with 

 respect to the rate of change of current in the wire, will 

 commence the breakdown of the gas, thus allowing a less 

 E.M.F. due to the magnetic induction to complete it. 



To put this to the test, a single turn of wire was put round 

 a bulb and the spark-gap adjusted so as to give a very faint 

 or no luminous ring ; on the top of the bulb was laid a piece 

 of tinfoil connected to one pole of a J in. spark induction- 

 coil ; when the coil is worked the tube is filled with a faint 

 glow : if now the Leyden jars are charged and discharged 

 there will be sometimes a ring in the bulb which will be 

 occasionally quite bright. The reason it cannot be always 

 bright is of course that the discharges of the induction-coil 

 are periodic, as are also those of the jars, and it is only when 

 the two are properly timed (i. e. the P.D. due to the coil 

 coming either just before or simultaneously with the spark) 

 that there will be a bright ring. 



This experiment seems to settle the question and show 

 conclusively that a properly timed electric stress in the bulb 

 due to an electrostatic field will allow an E.M.F. due to the 

 alternating current in the wire to produce a breakdown of 

 the rarefied gas, which the latter is too small to effect without 

 the aid of the former*. 



In Expt. 1, when A and C are connected this field will 

 exist between C and b the side of the turn of wire remote 

 from 0, and must therefore pass through the bulb. When A 



* To prevent misconception, it had better be definitely stated that this 

 electrostatic stress does not necessarily act in the same direction as the 

 E.M.F. due to the rate of change of magnetic induction. In experiments 

 (1) to (5) the direction of the former will be through the bulb from side 

 to side, while that of the latter is a circle coplanar with the wire. As 

 the discharge in a gas is of a nature more or less electrolytic, being 

 accompanied by the splitting up of the molecules, it seems reasonable to 

 suppose that anything which increases the number of dissociated molecules 

 will enable a smaller stress to produce a breakdown in the form of a 

 luminous discharge. 





