THE THERMOGRAPH: ITS EVOLUTION AND DESTINY. 681 
circuit. The strip of paper passing over the brass cylinder, having been satu- 
rated with a solution of chloride of sodium, pyrogallic acid, and ferrocyanide of 
potassium, the instrument is complete. 
When this combination is in operation, a current of electricity will pass from 
one pole of the battery to the binding-post (A) of the thermometer proper, 
through the substance in the vulcanite tube to emerge at binding-post (B) ; thence 
_ through the helix to the lever, along this to the silver stylus; thence through the 
moistened paper and brass cylinder to the other pole of the battery—thus com- 
pleting the circuit. Upon the application of varying degrees of heat to the ther- 
mometer proper, the resistance the current meets with during its course will be 
varied in precise accordance with the various changes of temperature. This 
waxing and waning current will now pass through the helix, and by the latter’s 
peculiar action produce to and fro motions in the lever, passing, at the same time, 
through the lever and chemically prepared paper, and producing as it passes a 
double chemical decomposition upon the paper; one of which decompositions 
renders the development of friction, during the movements of the lever, so slight 
as to be imperceptible; the second decomposition producing a change in color 
upon the paper, corresponding to the movements of the stylus, and affecting no 
larger surface than it covers, thus obviating the additional friction accompanying 
the use of an ordinary marker. 
From this description you will understand that the lever is moved backward 
and forward by a difference in the attractive power exerted by the helix, this in 
turn being dependent upon the strength of the current, which has already passed 
through the thermometer proper, and there been moulded into electric waves 
corresponding to the heat waves; the motion of the lever being facilitated by the 
lubricating action of the current, as the result of one of the chemical decomposi- 
tions during its passage through the chemicaily moistened paper; while the other 
decomposition causes a discoloration, and thus produces a mark corresponding 
in outline to the movements of the lever. This mark will, therefore, form an 
irregular line, whose sinuosities will give a graphic representation of the heat 
variations. This apparatus is extremely sensitive and can be made to record 
I-100 of a degree. 
Now, after marking upon our strip of paper the minimum and maximum 
points representing respectively go° and 110°, it becomes a very easy matter to 
determine the degree of heat represented by any point lying within this range. 
This is accomplished by dividing the intervening space into any number of equal 
parts, when any one of these divisions will represent a degree or any part of a 
degree, according to the number of divisions. These horizontal lines may be 
placed at such distances from each other as to represent 1-10 of adegree. Having 
provided the traveling paper with a uniform speed, it also becomes an easy matter 
to determine the time represented by any given distance upon its surface; for, 
supposing a certain amount of paper passes a given point in the instrument in 
one hour, to determine the amount passing the same point in five minutes, it is 
IV—46 
