100 Mr. W. Duddell: Instruments for the Measurement 
same P.D. at high frequencies. Thus at 100,000 ~ per second 
a certain sensitive ‘‘ Ayrton-Mather ” electrostatic voltmeter, 
which had applied between its terminals a P.D. of 3°6 volts 
and gave a deflexion of 392 scale-divisions, was found to 
have an alternating current of 0:12 x 10-? ampere flowing 
through it due to its capacity. In order to produce the same 
angular deflexion on the thermogalvanometer, a current of 
about 0°8 x 10-* ampere was required, so that for the same 
sensibility, considered as voltmeters, the thermogalvanometer, 
a hot wire instrument, only required less than 7 times the 
current taken by a sensitive electrostatic voltmeter, at a 
frequency of 100,000 ~ per second. Later modifications of 
the instrument have actually reversed the positions, so that 
the thermogalvanometer can be made to take less than one- 
third the current required by the electrostatic voltmeter to 
give the same deflexion. 
The sensibility and the relative calibration depend on the 
resistance and exact position of the heater A. It is evident 
that if it is required to measure a very small current, then 
the heater must have as high a resistance as possible; con- 
versely, to measure a low voltage h should have a low re- 
sistance, so that for any given purpose there is a_ best 
resistance for the heater. 
The power dissipated by the heater to work the instrument 
does not depend much on the resistance of the heater. 
Owing to the fact that the heater must be small in order 
that the instrument may be quick in action, I have usually 
used a straight wire about 3 or 4 mm. long, as shown at h, 
fig. 4, or sometimes a single loop. It was easy enough to 
make heaters of this type having a resistance under 5 ohms, 
but for higher values great difficulties were experienced. 
Straight heaters 3 mm. long, having resistances up to 20 ohms, 
were finally made by cutting very narrow strips out of gold- 
leaf. In this way I have made strips of gold 1 inch long 
having a resistance over 100 ohms. The manufacture of 
still higher resistances presented great difficulties, until lately 
I have succeeded in platinizing glass threads and soldering 
them in place. By this means it is quite simple to make 
heaters having resistances up to 1000 ohms for 8 mm. length 
and still higher values can be made. These resistances will 
easily carry sufficient current for the present purpose; a 
2000 ohm heater stood 2 milliamperes without damage, but 
was destroyed by 6 milliamperes. It remains, however, to 
be seen whether they deteriorate with time, up to the present 
the results with heaters under 1000 ohms seem satisfactory. 
A set of these heaters have been made and put in the above 
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