This is a very sensitive galvanometer, patterned after one built by Lord Kelvin. This type galvanometer is capable of detecting a current as small as 10 picoamperes for a deflection of 1 mm at a distance of 1 m. Within each of the two brass cylinders are two facing coils, and in the center of the cylinder, betrween the coils, is a small piece of mica with five short magnetized needles glued to it. On each mica flake the needles are all turned the same way, but the two sets are oppositely directed in order to minimize the effect of the earth's magnetic field. The two sheets of mica are rigidly attached together and to a mirror between the cylinders, and mica and mirror are all suspended by a fine fiber. ("Astatic" refers to the fact that the design minimizes the effect of the earth's magnetic field.) This instrument was purchased from Queen & Company for $30, probably near 1900.
A sensitive galvanometer designed by William Thomson (later Lord Kelvin) in 1858 to detect the current through the Atlantic cable. A small mirror is suspended by a thread between two coils, and on the back of the mirror are glued several short and light magnets. The curved bar over the galvanometer case is a permanent magnet which can be rotated and raised or lowered to minimize the effect of the earth's magnetic field and to center the supension system. The instrument was purchased from Queen & Co. for $30, but the date is not known.
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.
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.
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.
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.
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.
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.
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.
This is a standard capacitor used to calibrate equipment and measure other capacitances.
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.
The base of this tangent galvanometer is marked "Queen & Co., Makers, Phila." The date of purchase is not known.
This variable standard of self-inductance is graduated in milli-henries.
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.
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.
This demonstration electric motor was purchased from Queen & Company, but the date is not known. Probably it was before 1900.
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.
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.
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.
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.
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.
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.
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.
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.