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Simple Astatic Galvanometer
Astatic means that the instrument is constructed in a way that minimizes the effect of the earth's magnetic field. Electrical current being measured passes through coils and produces a magnetic field which causes the magnetized needle suspended above the card to rotate. Not visible is a second needle, rigidly attached to the one above, which hangs below the card in a region where the magnetic field is the opposite of the field above. That needle has the opposite polarity from the one above, so that the earth's field acting on both needles has negligible effect.
Decade Bridge
This decade bridge is a form of Wheatstone bridge, a device to measure electrical resistance. This bridge provides two of the four arms of the complete Wheatstone circuit, and the unknown and a standard resistor had to be added. A battery and galvanometer were also attached externally.
Demonstration Electric Motor
This demonstration electric motor was purchased from Queen & Company, but the date is not known. Probably it was before 1900.
Induction Coil and Interrupter
This piece of equipment dates from at least 1846. It was brought when the college moved from Davenport to Grinnell in that year.
This is an induction coil with a core made of iron wires--perhaps knitting needles. The primary circuit included the base which looks like part of rasp and an iron bar on the end of a wire. When the iron bar was dragged over the toothed base, the primary circuit was broken and remade many times quite rapidly. The rapid start and stop of the primary circuit induced a current in the secondary winding.
Helixon Stand
In a listing of physics equipment from around 1900, this device is called a "Helixon Stand." The term "helix" was used to describe a coil of wire through which an electrical current could be passed in order to magnetize a piece of steel passing through the coil. This appears to be such a helix, and perhaps Helixon was a trade name.
Lamp with Scale
Before electric lights were available, oil lamps such as this one were used for illumination in the laboratory. The motion of the mirror on the movable part of a galvanometer was observed either by looking with a telescope at a scale reflected in the mirror or by observing the movement of a spot of light reflected from the mirror onto a scale. This lamp and scale were used for that purpose--light from the lamp passed through a narrow slit to the mirror, and then it was reflected back onto the scale.
One True Ohm
This is a high precision resistor used in a Wheatstone bridge to measure electrical resistance. The label on the resistor says that it is "One true ohm at 18.4 degrees C." The two rods would have dipped into pools of mercury to insure good electrical connections.
Post Office Bridge
This is a Wheatstone bridge designed to locate a short circuit in a telephone or telegraph line by measuring the resistance to the short. It gets the name "post office bridge" from its adoption by the British Post Office, which operated telegraph and telephone systems in addition to delivering mail. The upper row of plugs determine a ratio and the lower rows are a standard resistance.
Hand-Driven Generator
This hand-driven generator was made by Palmer & Hall of Boston. Turning the wheel rotates two coils wound about iron cores which are at the ends of a horseshoe permanent magnet.
Standard Resistor with Mercury Cups
This resistor is exactly 10 ohms at a temperature of 23.7 degrees C. It was purchased in 1892 for $5.00.
Connection to a circuit was made through mercury in the cups into which the heavy leads from the resistor dip.
Two Inductors
These two standard inductors, marked 0.1 Henry and 0.01 Henry, were purchased by Professor Almy shortly after legal standards were adopted in 1893. Both were made by Hartmann & Braun A.G., Frankfurt A.M. They are numbers 245 and 246.
Astatic Galvanometer with Glass Dome
A galvanometer is a device for detecting electric current. The galvanometer pointer deflection is from electric current passing through a coil immersed in a magnetic field from permanent magnets.
Galvanometer with Three Horseshoe Magnets
These two galvanometers each have three horseshoe magnets. The coil beside each magnet should be suspended vertically between the poles of the magnets.
