double revolving magnet
Date: circa 1842
Inventory Number: 0130a,b,c
Classification: Electromagnetic Apparatus
Attributed to: Daniel Davis, Jr. (1813 - 1887)
Cultural Region:
Place of Origin:
Dimensions:
22.4 × 11.2 × 11.2 cm (8 13/16 × 4 7/16 × 4 7/16 in.)
Bibliography:
Davis's Manual of Magnetism
Daniel Davis, Jr. and the Elecromagnetic Instrument Industry: A Field with Great Potential in the Early 19th Century
Description:
From an upright pivoted post, two iron U-shaped electromagnets (facing each other) make a ring. They are wound with green painted wire. A pole-changer is associated with each coil (though badly damaged here). The base has two binding posts, which were connected to a battery.
This instrument bears two old Harvard numbers: "7-14" and "6-3-18".
This instrument was sold between $6.00 and $10.00 by Davis in 1842.
This instrument bears two old Harvard numbers: "7-14" and "6-3-18".
This instrument was sold between $6.00 and $10.00 by Davis in 1842.
Signedunsigned
FunctionTo show how an electromagnetic ring spins when a battery connection is complete.
This is how Davis explained the phenomenon in 1842: "The cups C C being connected with the battery, the current will flow along one of the wires W W, to one of the silver rings secured to the axis at D, thence through the wire enveloping half of the lower electro-magnet, to one of the springs playing on the pole-changer at P; it then traverses the wire surrounding the upper electro-magnet, with which the pole-changer is connected. Descending now to the opposite spring at P, it circulates around the other half of the lower semicircle, and thence back to the battery.
By this means the poles of the upper semicircle are revered twice in each revolution, while the polarity of the lower one remains unchanged. The upper electro-magnet will consequently rotate in the same manner as those in the instruments we have just described, while the lower one will move in the opposite direction, on the principle of reaction; its own poles being of necessity attracted and repelled with equal force while they are attracting and repelling those of the upper one. It would revolve as rapidly as the other, were it not that the friction of its axis is doubled in consequence of sustaining the weight of both electro-magnets. By holding the other stationary, however, the lower one will acquire a considerable velocity, which it will retain for a while when its fellow is release; their rapid motion causes them to present the appearance of a hollow sphere."
This is how Davis explained the phenomenon in 1842: "The cups C C being connected with the battery, the current will flow along one of the wires W W, to one of the silver rings secured to the axis at D, thence through the wire enveloping half of the lower electro-magnet, to one of the springs playing on the pole-changer at P; it then traverses the wire surrounding the upper electro-magnet, with which the pole-changer is connected. Descending now to the opposite spring at P, it circulates around the other half of the lower semicircle, and thence back to the battery.
By this means the poles of the upper semicircle are revered twice in each revolution, while the polarity of the lower one remains unchanged. The upper electro-magnet will consequently rotate in the same manner as those in the instruments we have just described, while the lower one will move in the opposite direction, on the principle of reaction; its own poles being of necessity attracted and repelled with equal force while they are attracting and repelling those of the upper one. It would revolve as rapidly as the other, were it not that the friction of its axis is doubled in consequence of sustaining the weight of both electro-magnets. By holding the other stationary, however, the lower one will acquire a considerable velocity, which it will retain for a while when its fellow is release; their rapid motion causes them to present the appearance of a hollow sphere."
Curatorial RemarksLater editions of Davis's book on magnetism (for instance, 1848, fig. 152, p. 218) show only the other design with straight electromagnet and a square base.
Primary SourcesDaniel Davis, Jr., Davis's Manual of Magnetism (Boston: Daniel Davis, Jr., 1842), 106-107.
ProvenanceFrom the Department of Physics, Harvard University.