Alternath\<f Curventii to Cid'ihration of CapacUy-hoxes. 715 



An ordinary '" quart " leyden-jar was compared with the 

 air-leyden with the following results. 



Table IV. 



c... 



The Kelvin 

 , J air-leyden 5030 



leyden-jur. ^ , 'm.F. , 



Quart 



I 



55^ 



It will thus be seen that two leyden-jars can be compared 

 with an accuracy approaching -^q per cent. 



By constructing an air-condenser, preferably in the form of 

 two concentric tubes, as recommended by Prof. Fleming, and 

 comparing its capacity with that of a standard condenser, 

 first with air as the dielectric, and secondly with another 

 medium replacing the air in the condenser, it is possible to 

 find the specific inductive capacity of that medium. 



It will, we think, be admitted that the method of comparing 

 capacities described is one of great convenience and accuracy. 

 It is as easy to carry out as an ordinary Wheatstone's-bridge 

 measurement of resistance, and now that alternating currents 

 are so easily procurable, we hope that the method will be 

 found useful in the workshop as well as in the laboratory. 



Comparison of Small Capacities. — For capacities of the 

 order of a few M.M.F.''s the Wheatstone's bridge is not 

 so suitable. In this case it is preferable to join the capacity 

 up in series with the moving coil, and to put the whole 

 pressure on the coil. To get increased sensitiveness we 

 constructed the coil C, described above. A \Qvy rough 

 air-condenser was made of two sheets of tin plate 10 cms. 

 X 10 cms., separated by three small pieces of ebonite 0"5 cm. 

 thick. The capacity of this is approximately 18"8 M.M.F.'s. 

 The coil was deflected through an angle equivalent to 6° 

 after magnification in the microscope. The same tin plates 

 separated by ebonite l"o cm. thick (capacity = T'l M.M.F.^s) 

 gave a deflexion equivalent to 2°*4. It was estimated that 

 ^ of this, i. e. 0*3 3I.M.F., would have produced an 

 observable effect. In this case the lag in the current ex- 

 pressed by tan~ML^^— -r^WR, is almost exactly — 7r/2, for 



though L, the self-induction of the moving coil, is quite 

 appreciable, yet its effect is completely drowned by the 



exceedingly large value of -r-, . 



