110 BELL SYSTEM TECHNICAL JOURNAL 



expected to display the most complete symmetry and smoothness. 

 F. Hund tried to devise an atom such that any one-volt electron pass- 

 ing within a distance r of the centre would be deflected through 

 exactly 360° before coming out; he attained a formal solution of the 

 problem, but the model involved a continuous distribution of negative 

 charge from the nucleus outward to the distance r, which is quite 

 incompatible with all our other knowledge of atomic structure. H. A. 

 Wilson of Rice Institute tried out a well-known and popular model, 

 consisting of a nucleus surrounded by a spherical surface of radius r, 

 over which negative charge equal in amount to the positive charge 

 on the nucleus is uniformly spread. For very slow electrons, the 

 average angle of deflection is 90°; it increases with speed, becoming 

 180° at a certain critical value v at which every electron is turned 

 back into the direction whence it came; beyond vo it decreases in- 

 definitely with increasing speed. At v the oncoming electrons are 

 more radically deflected by the atoms, so to speak, than at any greater 

 or lesser speed; below Vo the variation of mean deflection with speed 

 is in the proper sense to agree with experiment, but not by any means 

 of a sufficiently great order-of-magn.itude. The theory, however, 

 seems to explain the mild variations encountered in such gases as 

 hydrogen and nitrogen, and the explanation of the more striking 

 ones may lie in the same direction. 4 



Important contributions have lately been made to our knowledge 

 of self-sustaining discharges, such as the glow and the arc, which 

 maintain themselves as long as the proper voltage is applied at the 

 electrodes, without requiring the assistance of a separate source of 

 ions such as a hot filament or an outside ionizing agency such as X- 

 rays. The field has perhaps been somewhat neglected, because it is 

 easier to obtain simple clear results with electrons and ions admitted 

 into a very rarefied gas after being generated elsewhere. In a self- 

 sustaining discharge, there is usually a sudden steep potential-drop 

 just in front of the anode, and another just in front of the cathode — 

 the so-called anode-fall and cathode-fall; in the region between, the 

 potential varies gradually. The anode always tends to become very 

 hot, and Gunther-Schulze at Berlin has lately measured the rate at 

 which heat is generated at the anode of a mercury arc; he finds that 

 it agrees wonderfully well with the rate calculated from the assump- 

 tion that practically the entire current is carried by negative ions 



4 Any competent theory must explain the results obtained by different methods, 

 notably the fact that the value of r measured in an apparatus like Ramsauer's 

 agrees with the value measured in an apparatus in which electrons deflected through 

 considerable angle should yet reach the collector, and so be counted as though 

 they had not been deflected at all. 



