of Large and Small Alternating Currents. 97 
By observations of this kind it is often possible to trace the 
eauses of alternators not operating properly in parallel and to 
find which particular defect and which engine is the cause. 
Figs. 2 and 3 (Pl. III.) are two records obtained on a 
circuit in which there was purposely produced a cyclic irre- 
gularity having a known wave-form. In each case the cyclic 
change on the voltage was about +1 per cent. from the mean. 
In fig. 2 the voltage was changing sinusoidally about 121 ~ 
per minute, and in fig. 8 it was changing along a square wave- 
form about 68°5 ~ per minute. In this latter record the shape 
of the curve of growth and decay of deflexion and the conse- 
quent rounding off of the corners of the curves are well shown. 
In both records 1 cm.=1 per cent. change in voltage and 
10 em.=1 second. 7 
The second instrument is much more delicate and sensitive. 
It was made primarily to measure very small voltages and 
currents even with very high frequencies up to 120,000~ 
per second. At this high frequency the alternator I made 
would only give an extremely small output ; it was therefore 
necessary, in order to carry out the experiment, to reduce 
the power required to work the instrument to its lowest 
possible value. I have called this instrument a Thermo- 
galvanometer. 
The principle of the thermogalvanometer is quite simple. 
It consists of a resistance which is heated by the passage of 
the current to be measured, and the radiant heat from which 
falls on the thermojunction of a Boys:radio-micrometer*. As 
at first constructed, it consisted of a heating-resistance made of 
three or four turns of 0°001 inch diameter platinum-silver wire 
wound on a piece of mica, and placed as close as possible to 
the radiation receiving-plate of an ordinary Boys radio- 
micrometer, made by the Cambridge Scientific Instrument 
Company. This instrament was sensitive but exceedingly 
slow in action, taking over a minute to approximately attain 
the deflexion corresponding with the current flowing. 
A new radio-micrometer was therefore constructed fol- 
lowing the instructions given in Professor Boys’ paper, but 
having a very much smaller suspended loop than the radio- 
micrometer as usually constructed. The radiation receiving- 
plate was also omitted in order to reduce the quantity of 
metal to be heated, and the heating resistance was placed 
directly underneath the lower thermojunction, so that the 
junction received heat from it both by radiation and convection. 
The arrangement of the instrument is shown diagrammatically 
~* Philosophical Transactions of the Royal Society, 1889, vol. clxxx. 
p 159. 
Phil. Mag. 8. 6. Vol. 8, No. 43. July 1904. H. 
