Ratio of the Electromagnetic to the Electrostatic Unit. 321 



May 22. Bridge reading 24,430. Temperatures : Graphite 

 19°*8, A = 20°'3, B = 20°*6. 



9AA*n-f 2000 ° from B = 20,012 at 20°= 20,01 5 at 20°-6 

 J4,4^u-j 443Q ^ A= 4443 ^ = 4444 ^ 20°-3 



24,459 

 24,45 9 x 99-89 = 2,443,200 ohms at 19°- 8 = temp, at which used. 



This value is reliable to within one part in five thousand. 



It is proper to add that if these graphite resistances are put 

 into a circuit where there is a large difference of potential 

 between their terminals, their resistance is immediately 

 diminished by heating. With three Bunsen cells used in 

 measuring their resistance no heating was perceptible. In 

 the determination of capacity there was a difference of 

 potential between the terminals always less than two volts, 

 and usually less than one. When the temperatures were 

 maintained constant, the resistance after use was always pre- 

 cisely the same as before. While, therefore, the use of high 

 graphite resistances is somewhat restricted where greataccuracy 

 is desired, they still may serve a very useful purpose in many 

 cases, and are the most convenient and reliable of any high 

 resistance, aside from metal wires, that I know anything 

 about. 



Arrangement of the Apparatus. 



The vibrators were fixed as near as possible to the condenser 

 to reduce the capacity of the charging wires to a minimum. 

 The condenser, galvanometer, and other parts of the apparatus 

 were insulated with great care, and yet in spite of all pre- 

 cautions leakage made its appearance on rainy days, and a 

 slight trace of leakage could usually be detected. Observa- 

 tions were consequently confined to fair weather. The 

 apparatus for the determination of the frequency of the forks 

 was always ready for use. 



Sources of Error. 



1. Resistances. — The constant errors in the resistances must 

 have been very small, and corrections were always carefully 

 made for temperature-fluctuations. 



2. Tuning-Forks. — Michelson's method furnishes a very 

 exact determination of the period of an electric tuning-fork ; 

 but unfortunately the period does not remain constant. This 

 is especially the case with the higher fork, the charging wires 



