WITH ELECTROMAGNETIC FORCE. 



135 



I took care to make the observations without touching the micrometer, and 

 took advantage of the attraction to check the oscillations of the disk. The 

 experiments in which these precautions were taken agree together as well as 

 I could expect, and lead me to think that, with the experience I have acquired, 

 still better results might be obtained by the same method. It must be borne 

 in mind that none of the results were calculated till after the conclusion of all 

 the experiments, and that the rejected experiments were condemned on account 

 of errors observed while they were being made. 



Any leakage arising from want of insulation of the fixed disk would 

 introduce no error, as the difference of potentials between the two disks is 

 measured by the current in the galvanometer, through a known resistance, 

 independently of any leakage. 



All that is essential to accuracy is that the position of equilibrium before 

 making contact should be at true zero, the same as when there is no electrical 

 action, and that this equilibrium should not be disturbed when simultaneous 

 contact is made with both batteries. 



Experiments on May 8. S' = 1710 Ohms. 



Mean value of v = 28798 Ohms, or B. A. units, 



or 288,000,000 metres per second, 

 or 179,000 statute miles per second. 



The " probable error " is about one-sixth per cent. 



* In experiment 8 Mr Hockin and I changed places. 



