495 



NA TURE 



[March 22, 1906 



this system than they were in the previous one. For in 

 the first case the system B got into the condition in which 

 it radiated as much energy as it received, and thus did 

 not absorb any of the energy ; in the second case, how- 

 ever, B became luminous before its radiation was equal 

 to the .ibsorption ; it is thus taking in more energy than 

 it gives out, and this may result in a diminution in the 

 rate of supply of energy to A. It would be so, for ex- 

 ample, to a marked extent if the conditions were such that 

 A received a considerable portion of its supply of energy 

 from B ; this diminution in the supply might be great 

 enough to prevent the internal energy in B reaching the 

 critical value. Thus the effect of the increase on the rate 

 of supply of the electrical energy might be to weaken, or 

 even obliterate, the lines of A, and while with the smaller 

 rate we had the lines of A and not those of B, with the 

 larger rate we might have the lines of B and not those 

 of A ; thus an increase in the rate at which the electric 

 field is doing work surh as would be produced by in- 

 creasing the current through the discharge tube might 

 result in an entire change of the spectrum. We should 

 expect that it would only be in exceptional cases that the 

 lines of A would be obliterated under the conditions hold- 

 ing in case 2, but in all cases the increase in the brilliancy 

 of the lines of B would be large compared with the in- 

 crease of those in A. 



We see from the equations giving E, and E, that until 



the supply of energy has lasted for a time comparable 



witli i 0„, E 2 is large compared with E, ; thus for electrical 



- which last for an exceedingly short time we 



might easily have the lines "1 B visible and nut those 

 of A. 



In a discharge tube conveying an electrical current the 

 amount of work per unit volume of the gas done by the 

 electrical forces per unit time varies very largely from 

 one point of the tube to another ; if the cross section of 

 the discharge is the same at all parts of the tube, so that 

 the current density is uniform, the rale at which the 

 electrical forces do work will be proportional to the electric 

 force ; as this is much greater near the kathode than at 

 other parts of the tube, we should expect the lines of 

 systems of the type B to preponderate near the kathode, 

 and t" be absent or much feebler in other parts of the 

 tube. If the tube were of the type frequently used for 

 spectroscopic purposes with a capillary portion in the 

 middle, then since the current density is much greater in 

 this portion than in any other, the rate of work per unit 

 volume of the gas will be much greater in the capillary 

 portions than in the wide parts of the tube, and we should 

 therefore expect the lines of systems of tin- type B to be 

 much more prominent in the capillary part than in the 

 wide part. 



Effect of Self-induction and Capacity. — Suppose that we 

 have a tube of uniform bore arranged as in Fig. 6, the 

 terminal of the tube being connected with the plates of 

 a condenser of capacity C, and that there is a coil the 

 coefficient of self-induction of which is L placed in series 

 with the tube; then if the discharge through the coil 

 begins when the potential difference between the plates 



NO. 1899, VOL. 73] 



of the condenser is V„, the potential difference between 

 the plates after a time t will be 



\\ cos pt, 

 and the current through the tube 



C\\p sin p t, 

 where p = if 4/hC. 



Thus the maximum value of the product of the current 

 and the potential difference, i.e. rate at which the electric 

 forces are doing work in the tube, is CV = /> or \* 2 \/C/L, 

 and is thus proportional to the square root of the capacity 

 and inversely proportional to the square root of the self- 

 induction. Thus increasing the capacity increases the 

 maximum rate of work, and therefore increases the 

 brilliancy of the lines corresponding to systems of the type 

 B relatively to those of type A, while inserting self- 

 induction in the circuit increases the brilliancy of those of 

 type A as compared with those of type B. If we suppose 

 that the " blue " spectrum of argon corresponds to a 

 system of type B, the red to a system of type A, we have 

 an explanation of the changes in the spectrum of this 

 gas, for by inserting capacity in the circuit we can change 

 from the red to the blue spectrum, while having got the 

 blue we can get back to the red by inserting self-induc- 

 tion. I have here a little model which is intended to 

 illustrate the way in which the red and blue spectra of 

 argon originate. It is based on the fact that when we 

 send a current of electricity through a circuit the current 



does not rise to its steady value instantaneously, but, 

 starting from zero, increases with the time in exactly the 

 same way as we have supposed the intrinsic energy in the 

 atom, i.e. the way represented by the curve in Fig. 4. 

 The quantity in the electrical case corresponding to the 

 radiation /3 is the resistance of the circuit divided by the 

 self-induction, while the quantity a is inversely proportional 

 to the self-induction. Thus a circuit with large self-induc- 

 tion and small resistance is analogous to the system A, 

 while one with small self-induction and large resistance 

 is analogous to a system of type B. Now my model of the 

 argon atom consists of two circuits, C and D, placed in 

 parallel. C has large self-induction and small resistance, 

 D has little self-induction but large resistance. An electric 

 lamp is placed in each circuit. If I supply energy in one 

 way, i.e. by continuous current, to the system, the red 

 lamp in C lights up. the blue lamp in D is dark, while 

 if fed by an alternating current the blue lamp shines and 

 the red is dark. It would be interesting to see whether 

 :is we gradually diminish the self-induction we get the 

 whole of the lines in the blue spectrum at once, or whether 

 1 he lines of this spectrum enter in groups one after the 

 other. I have tried somewhat similar experiments with 

 the hot lime kathode to see in a mixture of gases, mercury 

 vapour and air, which spectrum first appeared as the rate 

 of cluing work in the gas was gradually increase!. The 

 great difficulty in this determination is that w-hen once 



