I was recently thumbing through an old (1958) Dictionary of Physics by H. J. Gray when I came across the convectron. This is an instrument used to give an electrical indication of physical deviation from the vertical. I had never heard of it. It uses the fact that the convection (1 Across) cooling of a straight, fine wire is much greater when the wire is horizontal than when it is vertical. Therefore, if two wires, as shown in Figure 1, are heated by passing an electric current (7 Down) through them, then the cooling effect by convection will be the same, as long as they are inclined at the same angle to the vertical.
A convectron is designed so that its arms a1 and a2 are two equal lengths of wire, which, at the same temperature, have the same resistance. N is the bisector (27 Across) of the angle between the two arms. When N is vertical the two arms cool at the same rate when the heating current is switched off, so both have the same resistance. If the device moves one way or the other, the rate of cooling is different in each arm, and there is a corresponding difference in electrical resistance.
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