§S4 ANNUAL REPORT SMITHSONIAN INSTiTtJTiON, 1910. 



the plates by means of the battery B, and, if the droplet had received 

 a frictional charge of the proper sign and strength as it was blown 

 out through the atomizer, it is pulled up by this field against gravity, 

 toward the upper plate. Before it strikes it the plates are short- 

 circuited by means of the switch S and the time required by the drop 

 to fall under gravity the distance corresponding to the space between 

 the cross hairs of the observing telescope is accurately determined. 

 Then the rate at which the droplet moves up under the influence of 

 the field is measured by timing it through the same distance when the 

 field is on. This operation is repeated and the speeds checked an in- 

 definite number of times, or until the droplet catches an ion from 

 among those which exist normally in air, or which have been pro- 

 duced in the space between the plates by any of the usual ionizing 

 agents like radium or X rays. The fact that an ion has been caught, 

 and the exact instant at which the event happened is signalled to 

 the observer by the change in the speed of the droplet under the in- 

 fluence of the field. From the sign and magnitude of this change 

 in speed, taken in connection with the constant speed under gravity, 

 the sign and the exact value of the charge carried by the captured ion 

 are determined. The error in a single observation need not exceed 

 one-third of 1 per cent. It is from the values of the speeds observed 

 that all of the conclusions above mentioned are directly and simply 

 deduced. 



The experiment is particularly striking when, as often happens, 

 the droplet carries but one elementary charge and then by the 

 capture of an ion of opposite sign is completely neutralized, so that 

 its speed is altogether unaffected by the field. In this case the 

 computed charge is itself the charge on the captured ion. 



The measurement of the distance between the cross hairs, correct 

 to about 0.01 mm., is made by means of a standard scale placed ver- 

 tically at exactly the same distance from the telescope as the pin- 

 hole J). 



THE DEDUCTION OF THE RELATIVE VALUES OF THE CHARGES CARRIED BT A 



GIVEN DROPLET. 



The relations between the apparent mass^ m of a drop, the charge 

 Cn^ which it carries, its speed, t\ under gravity, and its speed tu under 

 the influence of an electrical field of strength F, are given by the 

 simple equation 



mq 



or e. 



¥en-77-ig 



"" F V V, / ^ ^ 



1 The term " apparent mass " is used to denote the difference between the actual 

 mass and the buoyancy of the air. 



