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demonstration apparatus, electric

demonstration apparatus, electric

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Description
This small electrical motor was probably made in the 1840s, and probably designed for classroom demonstrations. It features a U-shaped permanent magnet mounted on a wooden base. A non-magnetic brass arch connects the two poles. A brass connecting terminal with an insulated (ivory?) bushing and silver leaf spring is set in each pole. Connecting a battery to the terminals energizes the electromagnet mounted on a vertical post in the center of the device. The current from the battery flows through the leaf springs to the center post. The section of the center post where the leaf springs make contact is made with insulators so that, as the post rotates, contact between the post and the springs will be interrupted. That section, called a pole-changer, reverses the polarity of the electromagnet. The interaction of the magnetic fields surrounding the permanent magnet and the electromagnet causes the entire post to rotate. The pulley seen at the bottom of the post would be connected to another device, possibly a tachometer or force meter, by means of a thin belt.
Daniel Davis described the operation of a similar unit in his 1847 catalog: “On making connection with the battery, when the bar is at right angles to the plane of the magnet, it immediately acquires a strong polarity. Its north pole is then attracted by the south pole of the steel U-magnet and repelled by the north pole. The south pole of the bar, on the contrary, is repelled by the similar pole of the upright magnet, and attracted by its opposite pole. These four forces conspire in bringing the electro-magnet between the poles of the U-magnet. When it reaches this position, each segment of the pole-changer leaves the spring with which it was in contact, and passes to the other. As the bar is moving past the poles by the momentum it has gained, its magnetism is destroyed for a moment, and immediately restored in the opposite direction. Each pole of the bar is now repelled by that pole of the permanent magnet which it has just passed, and attracted by the opposite one; it consequently moves on, the polarity being reversed twice in each revolution."
Ref: Daniel Davis, Manual of Magnetism (Boston 1848), pp. 211-213.
Location
Currently not on view
Object Name
Demonstration Apparatus, Electric
Physical Description
brass (overall material)
iron (overall material)
wood (overall material)
Measurements
average spatial: 34.2 cm x 16.5 cm x 16.5 cm; 13 7/16 in x 6 1/2 in x 6 1/2 in
overall: 13 3/8 in x 6 5/8 in x 6 in; 33.9725 cm x 16.8275 cm x 15.24 cm
ID Number
1993.0087.05
catalog number
1993.0087.05
accession number
1993.0087
Credit Line
Stevens Institute of Technology
See more items in
Medicine and Science: Physical Sciences
Data Source
National Museum of American History
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