Philosophical Transactions/Volume 54/The Description of a New Hygrometer
Read Nov. 8,
Now it is plain, that as much as the pannel expands between F and G, so much will the pully G be removed farther from the pin F; and just so much will the cord DE turn the pulley G backward; and any point in the groove of the pulley H ten times as much, because it is ten times the diameter of G in the groove: and this motion will cause the cord IK to turn the pulley L (and draw up the weight N) ten times as much as the pulley G is turned. So that, if the pannel expands a tenth part of an inch, by moist air, the pulley L will be turned quite round: and half round if the pannel expands but a 20th part of an inch: As the air grows dry, the pannel contracts, and the weight M descends and turns all the pullies the contrary way.
The back of the plate AA (Fig. 2.) is screwed to the other side of the frame (Fig. 1.) so as the straight edge of the plate may be even with the uppermost side of the frame, and the center B (Fig. 2.) may be directly over the center of the pully L (Fig. 1.) on whose axis the index BC (Fig. 2.) is fixed. And as the pulley L is turned by the cord IK, the index will he moved on the plate, and shew the degrees of moisture or dryness of the air.
If the expansion and contraction of the panel be so great as to move the index beyond the limits of the degrees on the plate, this may be remedied by putting on a larger pulley at L. — But if not great enough, in very wet and very dry weather, to move the index through all the degrees on the plate, the pulley L must be made less in diameter accordingly.
N. B. In three or four years at most, a new pannel should be put into the frame: because, when the old one has been so long exposed to the air, it will almost cease to be affected thereby. And therefore, a large thick piece of deal should be kept in reserve for that purpose; and about the thickness of a card always planed off that side from which the new pannel is to be taken.
At G and M, there must be small knobs of some hard wood glued on the back of the pannel below the graduated plate, to make a proper thickness for holding the wires upright and fast on which the pullies G and M do turn: for otherwise, the wires would soon loosen in the pannel.