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15 August 2003

Weather Vanes

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published October, 1969.

The next project is submitted by Cleveland H. Hood of Middlesbrough in England, who makes a hobby of constructing meteorological instruments He calls attention to an inexpensive device for electrically synchronizing the rotary motion of two mechanically independent shafts. The device consists of two principal assemblies: a transmitter and a receiver.

The transmitter includes a rotatable shaft that carries two electrically insulated wiper arms spaced 180 angular degrees apart. The tips of the arms make sliding contact with a surrounding toroidal coil of resistance wire. A comparable shaft of the receiver supports a bar magnet, much like the needle of a compass, that is free to rotate inside a surrounding configuration of three interconnected solenoids spaced 120 angular degrees apart. Direct current is applied to the wiper arms. From three points spaced120 angular degrees apart on the toroidal coil leads connect to each of the corresponding solenoids [see above left].

Figure 4: The vane assembled. Click image to enlarge.

When power is applied to the transmitter, the resulting current divides into three parts. The amount and the relative polarity of each part depend on the points where the wiper arms make contact with the toroidal coil. These currents induce a corresponding pattern of magnetic flux in the region of space enclosed by the solenoids. The bar magnet aligns itself with this field. Rotation of the transmitter shaft redistributes the current and, in effect, rotates the magnetic field of the receiver. The bar magnet and its supporting shaft turn in step.

Hood writes: "Any reasonably handy craftsman can construct a version of this synchronizing device, but in Britain (and perhaps in the U.S.) the transmitter and receiver are available on the surplus market. Known as the 'Desynn System of Remote Indication,' they are manufactured for use with airplane instruments by Smiths Industries Limited, Kelvin House, Wembley, Middlesex, England.

"I picked up a set of Desynns in the surplus market for 16 shillings (about $2) for remote display of wind direction at my home weather station. The weather vane is on high ground some distance from the house. The lower end of the vertical spindle that supports the vane assembly is coupled to the Desynn transmitter. The receiver of the Desynn displays on a dial in my study the direction in which the distant vane is pointed.

Figure 5: Mounting of the weather vane. Click image to enlarge.

"The design of the vane is novel in that the moving parts float in a container of light machine oil. The spindle passes through and is soldered to the axis of a sealed cylindrical can that functions as a float [see above right]. The oil that supports the float is contained by a glass jar that also serves as the base of the instrument.

"I wedged the Desynn transmitter in the bottom of the jar and joined its shaft by a spline to the lower end of the spindle. A sleeve bearing in the lid of the jar supports the spindle vertically. The perforated lid of a round tin can was soldered to the spindle just above the sleeve bearing to act as a rain shield. The spindle attaches to the balance point of the horizontal arm that carries the vane.

"The buoyancy of the float was adjusted, by putting oil inside, to counterbalance the weight of the rotating assembly. The weather vane behaves as though it were weightless and nearly frictionless, as indicated by the fact that it responds to an almost imperceptible breeze. It has operated continuously without maintenance in weather of all kinds for more than a year."