996 = Prof. J. A. Fleming on a Hot-Wire Ammeter for 
of one kilowatt size or less for the determination of the 
power-factor on the high-tension side, say, at 2000 volts, 
the magnetizing current to be measured may be at most 
about 0°02 of an ampere, or less in proportion to the size of 
the transformer. 
The capacity of a small condenser or short length of cable 
or wireless telegraph aerial can be measured by means of 
a simple harmonic electromotive force, if we have the means 
of measuring a small alternating current and a high voltage. 
For if a simple harmonic electromotive force of frequency 
n=2/p, and R.M.S. value V, is applied to a condenser of 
capacity C microfarads, then the alternating current (R.M.S. 
value) flowing into it has a value of CpV/10° amperes, pro- 
vided there is no sensible resonance. 
If the frequency is about 80 so that p= 500 and if 
V = 2000 volts, then a capacity as small as 1/500 of a micro- 
farad can be measured in this manner, provided we have the 
means of measuring the voltage V and an alternating current 
of the above-named magnitude. This can of course be done 
by any form of electro-dynamometer adapted for measuring 
very small currents, but the hot-wire ammeter here described 
is much simpler to construct. 
The following form of hot-wire ammeter can be so made 
as to measure currents as small as two milliamperes, and is 
easily calibrated at the time of using it. 
The ammeter consists of a wooden box (AB, see fig. 1) 
104 cms. in length, 8 cms. in height, and 6 cms. in width. 
Big. 



The top of this box opens on hinges, and in the centre is fixed 
an achromatic convex lens / having a focal length of 10 cms. 
The front of the box is cut down to form a window, W, which 
is glazed with a sheet of thin transparent mica (see fig. 1). 
In the box is fixed a square rod of well-seasoned pine, a metre 
in length and 2°5 cms. in width and breadth. To each end 
