The Magnetometer
   A magnetometer is used to measure the strength of a magnetic field. A magnetic needle (missing in the photograph) is hung on a cradle suspended by a fine torsion fiber from the top of the apparatus. A glass or brass cylinder probably surrounded the lower part of the apparatus to keep air currents from blowing the needle.

   The square of the period of oscillation of the needle and fiber system is proportional to the magnitude of the magnetic field. The system is set up with the needle in the magnetic meridian, the needle moved slightly to the side, and the period of oscillation measured. The unknown magnetic field is then applied, with its direction either parallel or antiparallel to the horizontal component of the earth's magnetic field, and the experiment repeated. 

   The ratio of the squares of the two periods is equal to the ratio of the horizontal magnetic field of the earth to the horizontal magnetic field of the earth plus the unknown. From this the unknown magnetic field can be deduced.

   This magnetometer is at the Museum of Early Philosophical Apparatus at Transylvania University in Lexington, Kentucky. 

This magnetometer is in the collection of historical apparatus at Case Western Reserve University in Cleveland, Ohio.

   It is by the firm of Hartmann and Braun of Frankfurt, Germany.

   When I first looked at it, the two wood and glass covers were in place, and it was only after a time that I realized that hey served to provide protection against air currents blowing on the oscillating magnet.

   These two magnetometers are in the Greenslade Collection. The one at the right is unmarked, but the left-hand one is an unusual piece of non-surveying equipment made by Gurley of Troy, New York. 

   In both cases the magnet, the suspension holding up the magnet and the rotating cap for the top of the suspension are missing. 

The device below is a rather different form of magnetometer. It consists of a fixed, multiple-turn coil though which a long, slender permanent magnet (actually, three thin bar magnets placed end to end in a brass shell) can be shuttled. Stops limit the portion of the magnet that passes through the coil. The kick delivered to a galvanometer connected to the ends of the coil is proportional to the number of lines of force entering or leaving the magnetic field between the two rest positions of the magnet. The apparatus was made by the L.E. Knott Apparatus Company of Boston, but is not in their 1916 or 1921 catalogues. An analogous piece of apparatus is described in Millikan and Mills' text, Electricity, Sound and Light (1908) in which the magnet is stationary and the coil moves between the stops. In the 1940 Cenco catalogue this is described as a "Distribution of Magnetism" apparatus; it is ascribed to H.A. Rowland of Johns Hopkins, and cost $13.50. This apparatus is in the Greenslade Collection.

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