THE THERMOGRAPH: ITS EVOLUTION AND DESTINY. 679 
the instrument in its proper position in the axilla. The composition used in the 
vulcanite tube (T) may be either a solid stick of baked lamp black, a series of 
thin carbon discs with intervening ones of silver, or a powder made of plumbago, 
gas-carbon and silver, finely divided. After receiving a communication from 
Thomas A. Edison in regard to this matter, I commenced a series of experiments 
to determine the most suitable compo-ition for this purpose, and the best results 
were obtained from the powder already referred to. The salient feature of this 
instrument is the changing of its electrical resistance with pressure, and the ratio 
of these changes, moreover, corresponding exactly with the pressure, the latter, 
in turn, being dependent upon and in unison with the rise and fall of tempera- 
ture. 
Here, then, was the true solution, for, by subjecting this instrument to vary- 
ing degrees of temperature, the resistance of the powder would vary in precise 
accordance with the pressure exerted by the uniform expansion of the spiral 
spring under equal increments of heat, and consequently a proportionate varia- 
tion would be produced in the strength of the current. The latter would thus 
possess all the characteristics of the heat waves, and by its reaction through the 
medium of some electro-magnetic piece of mechanism yet to be devised, these 
might be transferred to our movable surface, in the form of a sinuous line, whose 
rising and falling inflections would give a graphic representation of them. 
Now, that I had satisfactorily reduced this portion of the problem, the next 
in order was the devising that part of the instrument intended for recording such 
variations as the other branch might be subjected to. This, I assure you, was no. 
easy task, but one requiring a mint of patience and tedious application. For, 
first—it must be simple; second—there must be established a permanent relation- 
ship between the first and second branches of the instrument, in other words, 
there must exist throughout a strict interdependence; third—in order that the 
electro-motive force required might be reduced as much as possible, it must be 
delicate ; fourth—to render the latter possible, friction must be practically reduced 
toaminimum. To carry you through the almost endiess and varied experiments 
necessary in developing means for meeting these indications would be as tiresome 
as it would be unnecessary. Hence, I shall confine myself to the result only. 
If a number of coils of insulated wire be wound around a hollow reel, there 
is formed what is known to electricians as a /cdvx. If this is now placed in an 
electric circuit and a current passed through its convolutions, it is temporarily 
constituted a magnet, the two ends forming the poles; so that it may be said to 
possess all the properties of a permanent magnet during the passage of the cur- 
rent. Moreover, if such a helix, mounted in a vertical position in such a way 
that an iron rod can be introduced into it from below, be connected with a battery, 
the iron rod will be at once drawn up into it and be sustained oscillating in its 
axis, even though the rod may weigh considerable. 
The depth the iron rod enters will also depend entirely upon the strength of 
the current and the amount of resistance offered by the iron rod. This principle 
