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.

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.

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

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.

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.

These two galvanometers each have three horseshoe magnets. The coil beside each magnet should be suspended vertically between the poles of the magnets.

Scalamp Galvanometer, Cat. No. 7890/S

Model 375