38 Professor Rutherford on Radio-activity. [Jan. 31, 



rays onlj causes a slight movement of the electrometer needle. By 

 applying, however, a voltage nearly equal to that required for a dis- 

 charge through the gas there is a very rapid movement of the needle. 

 On removing the radium there is no appreciable current through the 

 gas. On placing a source of a rays near the small opening in the 

 cylinder so that some of the a particles can be fired along the axis of 

 the cylinder, the electrometer needle does not move uniformly, but 

 with a succession of rapid throws with a considerable interval in 

 between. Each of these throws is due to the discharge produced by 

 a single a particle entering the cylinder, increased several thousand 

 times by the intermediary of the strong electric field. If a sheet of 

 paper which stops the a rays is placed before the opening, the electro- 

 meter needle at once comes to rest. The interval of time between 

 the throws is not uniform. This is exactly what we should expect if 

 the number of a particles entering such a small opening is governed 

 by the laAV of probability. On the average, a certain number of a 

 particles are fired through the opening per minute, but in some cases 

 the interval is less than the average, in others much greater. In 

 fact, by observing the intervals between the entrance of a large 

 number of a particles, we should be able to determine accurately the 

 "probability" curve of distribution of the a particles with time. For 

 purposes of measurements, the active material, in the form of a thin 

 film covering a small area, is placed in an exhausted tube connected 

 in series with the ionisation cylinder, and at a considerable distance 

 from the hole. The number of a particles entering the opening per 

 minute is counted, and from this tlie total number expelled can be 

 calculated. Preliminary measurements show that the number of a 

 particles expelled from a known weight of radium is of the same 

 order as the calculated value. When the measurements are completed 

 it should be possible to determine the charge carried l)y each a 

 particle, since the total charge carried by the a particles from 1 gram 

 of radium is known. In this way it may be possible to settle whether 

 the a particle is a helium atom or not. In any case, it is a matter of 

 some interest to be able to detect by its electrical effect a single atom 

 of matter, and so to determine directly with a minimum of assumption 

 the magnitude of some of the most important quantities in radio- 

 active phenomena. 



[E. R.] 



