370 ilir Southerns, Experimental Investigation as to Dependence 



It will be seen from the above results that a positive effect 

 is constantly exhibited, constant in direction and as uniform in 

 magnitude as can be expected when regard is had to the nature of 

 the experiment, and the fact that the effect only represents a 

 difference of a few ten thousandths of a milligramme. Indeed it 

 is surprising that the fluctuations are not much larger. Some 

 preliminary experiments, in which the sources of potential were 

 charged Leyden jars, gave similar results, though they were more 

 irregular on account of the leakage of the jars. They shewed, 

 however, that changes in the adjustment of the apparatus which 

 were constantly made, had no influence on the average values of 

 the results. 



From Experiment 8 J., i?, C it will be seen that the effect — 

 so far as measurements of the actual magnitude can be relied on 

 — is proportional to the induced charges or to the potentials of the 

 field plates. 



A few possible sources of error may be briefly alluded to : 



1. External electrostatic action due to lines of force entering 

 the balance case from outside. The fact that the front of the 

 balance case may be on or off (Experiment 1) or the windows 

 coated with tinfoil, or left bare without sensibly altering the result, 

 seems to shew that this source of error is inoperative. It is hardly 

 possible that it could be otherwise, for electrostatic effects de- 

 pending on the external conductors from key to balance would be 

 eliminated on reversal, and any permanent field which might exist 

 which could send stray Wnes on to the beam, would merely give a 

 permanent deflection which would not alter the value of 2S on 

 reversal of the fields F^, F^, unless these stray lines penetrated to 

 the surfaces of the end pieces where the induced charges resided. 

 An examination of the drawing will shew how unlikely this is to 

 be the case. Moreover the shape of the surfaces is such that very 

 few of the lines so reaching them would tend to deflect the beam. 

 If the result had been due to direct action of the cells, voltmeters, 

 or other parts of the electrical apparatus, other than the conductors 

 mentioned above, it would have changed sign in Experiment 6 

 when the balance connections were reversed, but this was not the 

 case. 



2. Mechanical operation of turning the reversing key. In 

 Experiment 6 the result would have changed sign had it been 

 caused by this operation. Moving the key produced no disturbance 

 of the balance. 



3. Accidental variations in the potentials applied. Actual 

 variations of four and eight volts made no perceptible difference 

 to the results. See Experiments S B, E, 4^ B. These variations 

 were much greater than any which could have occurred during the 

 majority of the experiments. 



