304 £. B. Rosa — Determination of v, the Ratio of the 



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 

 and spring contact having a varying eifect upon the rate in dif- 

 ferent adjustments. But the slower fork with mercury con- 

 tact 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 

 fork 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 micro- 

 scope 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 (fig. 1), th& 

 wires b, V (fig. 3) as well as the joining wire e have a certain 

 capacity which adds itself to that of the condenser when they 

 are connected, but which may be determined separately by dis- 

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

 H 5 and fast fork, the resistance a was 1874*5 and 153*0 respec- 

 tively in the two cases mentioned, which gives 1721*5 ohms 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 2*5 cm. 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 disconnected from 

 the ball and at a different potential ; but being lifted one or 

 two millimeters in disconnecting its capacity would be thereby 

 reduced. The effects of these two modifying 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 l-3500th of the 

 capacity of the condenser. No difference could, however, be 

 observed, although the trial was several times repeated. The 

 two effects have opposite signs, and if each is inappreciable 



