176 BELL SYSTEM TECHNICAL JOURNAL 



have to say about that question. Of the three physicists who have 

 announced experiments upon the problem, L. AI. Mott-Smith has given 

 the most impressive account. There are three tubes, of the type 

 exhibited in Fig. 12, their cylindrical chambers being some four 

 cm. wide. Two are in the same vertical line, 30 cm. apart; the third 

 moves to and fro along a horizontal line which intersects that vertical, 

 35 cm. below the lower of the two which are stationary. Triple 

 coincidences are counted; they are most frequent when all three of the 

 tubes are in line, as one would expect; when the movable tube is 

 shifted away from the line, the number declines, the curve of coinci- 

 dences N versus displacement x having "about the shape which is 

 expected from the geometry of the arrangement, assuming rectilinear 

 passage of the ionizing particles." 



There is another object in the vertical line beneath the lower of the 

 stationary tubes, which I have not yet mentioned: it is a piece of iron 

 15 cm. thick, which in some of the experiments is unmagnetized, in 

 some is magnetized in such a sense that charged particles which have 

 descended vertically through the upper counters must be deflected, 

 while remaining in that vertical plane which contains the horizontal line 

 along which the movable tube is being shifted. If the particles are 

 charged and of a single speed, the curve of A^ versus x should be shifted 

 along the axis of x; if they are electrons of energy 10^ (equivalent 

 volts), it should be shifted by 2.2 cm., and should be easily distin- 

 guished from the curve obtained when the iron was demagnetized. 

 Nevertheless, the two curves (or rather the three, for the iron was 

 magnetized first in one and then in the opposite sense, in the hope of 

 producing opposite deflections) are indistinguishable. Mott-Smith 

 considers that if the ionizing particles be electrons, their energy must 

 be not less than 2.10^ — a lower limit which may be halved, if they be 

 protons. These values however are contingent upon an assumption, 

 which I will mention in a moment. 



The others who have entered upon this problem are Curtiss and 

 Rossi. Rossi's first apparatus was remarkably like that which I have 

 just described, independently and far apart though he and Mott- 

 Smith were working — the one in Florence, the other in Texas. Later 

 he modified the scheme, having two counters only, and a pair of slabs 

 of iron so placed that when they are magnetized in one sense charged 

 particles passing through the upper counter and thence into either slab 

 should be deflected towards the lower counter, and when they are 

 magnetized in the other sense the charged particles should be deviated 

 away from the lower counter. Again the result was negative; and, 

 says Rossi, "the corpuscular rays are not deflected ... to such a 



