322 Mr. E. B. Rosa en the Determination o/v, the 



and spring-contact having a varying effect upon the rate in 

 different adjustments. But the slower fork with mercury 

 contact was not, even after making proper temperature- 

 corrections, perfectly constant. To avoid all uncertainty, and 

 obviate the necessity of applying a temperature-correction, the 

 rates of the forks were determined each time*anew, usually 

 before and after or in the midst of a series of observations on 

 capacity. As stated, the apparatus for the purpose was always 

 ready for use ; and without stopping the fork or changing its 

 circumstances in any way whatever, by simply closing the 

 clock-circuit and the primary circuit of the induction-coil, I 

 could in three to five minutes count a sufficient number of 

 flashes to give me the period of the fork true to within less 

 than one part in ten thousand. Occasionally a slight change 

 in the sound emitted by the fork, due to variation in contact 

 or current, suggested a possible change in the period ; a 

 moment's glance in the microscope would answer the question. 

 This method of dealing with the rates of the forks avoids the 

 introduction of small constant and large accidental errors, 

 which may happen when the rates are determined once 

 for all. 



3. Charging- Wires. — The vibrating armature p, the wires 

 6, b f , as well as the joining- wire e, have a certain capacity, 

 which adds itself to that of the condenser when they are con- 

 nected, but which may be determined separately by dis- 

 connecting the charging-wire at m. Thus, on April 15, with 

 R 5 and fast fork, the resistance a was 1874*5 and 153 respec- 

 tively in the two cases mentioned, which gives 172 1*5 as the 

 resistance corresponding to the condenser alone. This assumes 

 that the capacity of the charging-wire is the same when joined 

 to the ball as when separated. The capacity of the two and a 

 half centimetres of fine brass wire between the ball and the 

 shell (fig. 3) is nearly one per cent, of the capacity of the 

 condenser, determined experimentally. It would seem that 

 this capacity might be slightly greater when the wire was dis- 

 connected from the ball and at a different potential ; but being 

 lifted one or two millimetres in disconnecting, its capacity 

 would be thereby reduced. The effects of these two modi- 

 fying circumstances were separately very carefully studied. 

 With the rapid fork running very smoothly, a change of half 

 an ohm could be easily detected ; this would be equal to a 

 change of ^oo of the capacity of the condenser. No dif- 

 ference, however, could be observed, although the trial was 

 several times repeated. The two effects have opposite signs ; 

 and if each is inappreciable, much more would their sum be 



