602 Prof. J. A. Fleming on a Hot- Wire Ammeter for 
ammeter inserted so as to measure the alternating current 
taken by the jar. The high and low tension voltages were 
measured by Kelvin electrostatic voltmeters. The capacity 
of the jar or jars was calculated from the condenser-current A,, 
as read on the ammeter from the expression A=CpV/106, 
where V is the reading of the electrostatic voltmeter across 
the high-tension terminals of the transformer. The following 
Table II. gives the observations :— 






Tasce II. 
| 
Observed | Calculated 
Gia ree Terminal Voltages, eae p  |Condenser, Capacity in 
oneens<"* | of Transformer. zat =27n.| Current | Microtarads | 
ToT ail SEL male 
Jarno.1 ...; 975 | 13,750 13°7 463 | 00112 | 0-00176 
101°8 | 14,400 716-7 483 | 00121 000174 
Jarno.2 ...; 97:0 | 13,650 73:7 463 | 00107 000169 
101-7 | 14,250 16:7 483 | 00110 | 000159 
Jarsnos.1&2} 97:0 | 15,300 737 462 | 0:0204 | 0:00287 
in parallel. | 101°8 | 16,550 oes 483 | 00226 | 0:00285 






The capacities of the jars as measured by the rotating 
commutator described by the author and Mr. Clinton (see 
Proc. Phys. Soc. Lond. vol. xvii. p. 386, also Phil. Mag. 
May 1903, p. 493) were as follows :—Jar no. 1=0-00154 mfd., 
Jar no. 2=0:00124 mfd., Jars nos. 1 & 2 in parallel=0°0028 
mfd. 
Hence it will be seen that the hot-wire ammeter gave a 
capacity in close approximation to that given by the 
commutator method for the joint capacity of the jars, but 
that there was an apparent increase of capacity when the 
jars were measured separately by the ammeter method. This 
is clearly due to resonance. The changes in the transformation 
ratio of the transformer due to the addition of the capacity 
are apparent from the figures in Table IIT. Hence, in using 
this method, the precaution must always be taken of having 
sufficiently large capacity in parallel with the one to be 
measured to destroy resonance, but the current into the 
capacity, which it is desired to measure, is alone measured 
by the hot-wire ammeter. 
In connexion with this subject, I have had occasion to 
notice the manner in which even the small capacity of a 
Kelvin high-tension voltmeter will, in conjunction with the 
inductance of some transformers of high inductance, produce 
