r D is closed. On arriving
upstairs I pull the cord G, which causes the piece H to become
fixed in the vertical position by means of A. This opens the draft
door at the same time. Now when the furnace heats up sufficiently
it causes the air to expand in F, which causes the mercury in M to
rise a little above the point 2. This immediately causes a current
to flow through C which in turn draws R towards it, raises A and
causes H to drop to position X. This shuts the furnace door. Now
the furnace, of course, cools down, thus causing the air in F to
contract and consequently opening the circuit through C. If at any
time the furnace should overheat, the raising of A, on which is
grounded a wire from a signal bell upstairs, will make a circuit
through the bell by means of the point Z and wire leading
therefrom. This bell also serves to tell me whether H has dropped
or not. This same device of regulating the draft D can be used to
regulate the damper, found on the coal doors of most furnaces, by
simply fusing a platinum point on the other side of M and changing
the cord which is attached to D. A two-contact switch could also
be inserted to throw connections from 2 to 3. It would work in
this manner: The damper door, of course, which keeps a low fire,
would be up in a position similar to D; on the furnace cooling too
much, connection, due to contracting of air in F, would be made
through 3 and C, causing H to drop, thus closing door. This simple
device worked very well all last winter and gave me no trouble
whatever.

If you cannot readily procure a U-tube, you can make one, as I
did, and the work is interesting.

The U-tube is constructed in the following manner. A glass tube is
closed at one end. This is done by holding the tube in one corner
of a gas flame, somewhat near the dark area (A, Fig. 2), and
constantly turning the tube, when it will be found that the glass
has melted together. Now, after it is cool, about 3 or 4 in. from
the sealed end, the tube is held steadily so that the flame will
heat one small portion ( B, Fig. 2 ). After this small portion is
heated blow into the tube, not very hard, but just enough to cause
tube to bulge out. Allow to cool. Then reheat the small bulged
portion, blow quite hard, so that the glass will be blown out at
this point, forming a small hole. Now insert about 1/2 in. of
platinum wire and reheat, holding platinum

[Illustration: Making the U-Tube]

wire by means of a small plie