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Ballistic Galvanometer

Like many other galvanometers, a ballistic galvanometer has a coil which rotates between magnets. The ballistic galvanometer has the special feature that its rotating coil has a large moment of inertia. It is used to measure quantity of charge rather than currents, for the large moment of inertia permits the passage of a quantity of charge before the coil moves significantly. The passage of the charge produces an impulse, a momentary torque, which causes the coil then to swing slowly to some maximum position. Such a galvanometer was often used to standardize capacitors. This galvanometer was purchased from Queen and Company for $75. probably near 1900.

Bohnenberger Electroscope

This instrument almost certainly was on the Grinnell College campus before the tornado of 1882 which destroyed most of the college's scientific equipment. Its manufacturer is unknown. The Bohnenberger electroscope has a dry pile which produces a constant potential difference of the order of 1000 volts. Charge is placed on the electrode extending up from the top, whence it flows to a single sheet of gold leaf hanging between two plates attached to the dry pile. The charge on the gold leaf, hanging in the electrical field produced by the plates attached to a battery, caused deflection of the gold leaf. Because the polarity of the pile was known, the sign of the charge on the electrode and some indication of its magnitude could be seen.

Homemade Galvanometer

When Frank Almy came to Grinnell College in 1893 to teach physics, laboratory equipment was not as readily available as it was later. Probably the cost of equipment also placed a severe constraint on purchases. Almy made a number of galvanometers, some of which survive. One of those galvanometers is shown here. It had a mirror attached to small magnets suspended between coils.

Homemade Tangent Galvanometer

At the time Frank Almy began teaching physics at Grinnell College in 1893, the tangent galvanometer was an important part of the equipment of any physics laboratory. This is a tangent galvanometer that Professor Almy made. The date is not known, but it probably was before 1900. The compass in the center is a modern instrument; we do not know what compass Professor Almy used.

Epinius' Condenser

This is a capacitor with adjustable plates and a plate of glass which can be inserted between the plates. With static charges on the plates, the effects of plate separation and nature of the dielectric between the plates can be studied. This was purchased from Queen & Company, but the date is unknown.

Kohlrausch Wire

A Kohlrausch wire is a uniform wire wound on a form with a slider which moves along the wire. It can be used as part of a Wheatstone bridge or a potentiometer. This wire was made in Germany and imported by James G. Biddle. Its resistance is 195 ohms. The form is marble.

One Ohm Thermometer

This is one of several resistors of high quality which are in the museum. Such resistors were used as standards in bridge circuits to measure unknown resistances to high precision. The thermometer is included because the resistance is exactly one ohm only at a specified temperature.

One-Third Microfarad Standard

This is a standard capacitor used to calibrate equipment and measure other capacitances.

Post Office Bridge with Meter

This is a Wheatstone bridge designed to locate a short circuit in a telephone or telegraph line by measuring the resistance of the wire to the short. It is called a "post office bridge" because this design was adopted by the British Post Office, which operated telegraph and telephone services as well as delivered mail. This bridge, which is in a box, has a built-in galvanometer. The upper plugs determine a ratio and the lower rows are a standard resistance.

Tangent Galvanometer

The base of this tangent galvanometer is marked "Queen & Co., Makers, Phila." The date of purchase is not known.

Variable Self Inductor

This variable standard of self-inductance is graduated in milli-henries.

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.

Electric Egg

The electric egg is a glass vessel with an electrode at the top and another electrode at the bottom. At the bottom also is a tube with stopcock through which the air in the vessel can be pumped. It is used to demonstrate the production of light by a static electric charge. If the egg is evacuated and then a large static charge is place on the upper electrode, perhaps by connecting it to a static machine, a glow fills the egg. This was once suggested as a way to produce safe illumination in mines, but it was found that the glow does not persist long enough to be useful. This instrument was sold by Queen & Co. but probably was manufactured in Europe. The date is unknown.

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.

Seth Thomas Clock 11

The Seth Thomas clocks, acquired by the college in December of 1888, represent the pinnacle of accuracy in timekeeping within their era. The clocks have very heavy cast-iron frames and gold-plated regulators. The polished brass tubes on either side of the clocks house the weights which maintain the rhythm and accuracy of the clock regulators. The college purchased the clocks after receiving the gift of an 8-inch Clark refractor, along with a chronograph, micrometer and a transit telescope. Grinnell offered practical astronomy classes for the first time in the 1888-89 academic year. Located in Goodnow Hall and used in the Goodnow observatory , the clocks were set respectively for solar and sidereal time. This clock is set for solar time.The second image shows the Goodnow Hall clock room in the early 1900's.  The chronometer is on the table to the right. The clocks, routinely maintained by Kens Clock Repair of Brooklyn, Iowa. still keep excellent time and are on display in the Kistle Science Library in the Noyce Science Center.