Flip Coil #10112
A flip coil can be used to measure the magnetic field of the Earth and therefore it is also called an Earth inductor. A handle allows the experimenter to quickly rotate the coil 180°. The motion generates a temporary current in the coil and the total charge generated during the flip, usually measured by a ballistic galvanometer, can be used with data on the size and the number of turns in the coil to calculate the magnetic field intensity.
Reference: Max Kohl Catalogue No. 100 (c.1927) p.973.
This is a moving-coil ballistic galvanometer with a very small restoring torque used for measuring magnetic flux. It is calibrated in maxwells, an early unit of magnetic flux. It is used with an exploring coil in a closed circuit. The meter is set by mechanical means to zero while the coil is in the field to be measured. The coil is then removed from the field, causing a deflection on the meter, which yields the change of magnetic flux in the coil.
Reference: Frank A. Laws, Electrical Measurements, New York, 1938, p.112-15.
Bismuth Spiral #10658
Hartmann & Braun, Frankfurt
This is used to measure magnetic fields by the change in resistance of a loop of bismuth placed in the field. Since it is very compact, only about 1 millimeter thick and 20 millimeters in diameter, it can be used in very narrow spaces such as between a pole face of a magnet and an experimental apparatus in the field. D.B. Brace used this in his research on the effect of magnetic fields on polarized light. Brace knew the inventor of this device, Philip Lenard, from their student days together in Berlin.
References: Max Kohl Catalogue No. 100 (c.1927) p.959; D.B. Brace, Philosophical Magazine 48, 318 (1897) and 1, 493 (1901).
Tangent Galvanometer #10192
A tangent galvanometer is used to measure electric currents if the local magnetic field of the Earth is known. Conversely, if the current can be measured independently, one can determine the Earth’s field. The coil is oriented with its axis perpendicular to the Earth’s field so that the compass needle is in the plane of the coil. Then when the current in the coil is turned on, the compass rotates to an angle depending on the relative strength of the current-induced field and the Earth’s field.
Reference: Central Scientific Co. Catalog F, 1923, p.243.
Tangent Galvanometer #10098
Electric Manufacturing Co., Troy, NY
This beautifully made instrument is as much a work of art as a scientific instrument. The 1-meter coil is encased in a copper tube and the compass dial is silvered. There are also moveable trays to hold bar magnets for additional experiments.
Reference: Ellen Warren Faller and Barbara P. Moore, "Historical Scientific Instruments at Yale University," Rittenhouse 7, 106-107 (1993).
Tangent Galvanometer #10189
This massive tangent galvanometer has a tilting coil to provide different sensitivities. The force on the compass needle decreases as the coil is tilted since that decreases the component of the field in the plane of the needle. Such instruments were used in the 1870s to measure the large currents produced by power station dynamos.
Reference: John T. Stock and Denys Vaughan, The Development of Instruments to Measure Electric Current, Science Museum, 1983, p.17.
Sine and Tangent Galvanometer #10169
Siemens Bros., London
Claude Servais Matthias Pouillet (1790-1868) invented the sine galvanometer, which is similar in appearance to the tangent galvanometer, but has provision to rotate the coil about a vertical axis. One rotates the coil from the magnetic meridian position until it coincides with the vertical plane containing the deflected needle. The current is then proportional to the sine of the angle of rotation. It is more sensitive than the tangent galvanometer and allows the use of a larger needle.
Reference: John T. Stock and Denys Vaughan, The Development of Instruments to Measure Electric Current, Science Museum, 1983, p.16; Gustav Wiedemann, "Die Lehre von der Electricitat," 1895, pp. 278-280.
Dipping Needle #10564
The magnetic field of the Earth is not, in general, horizontal but in most of the Northern Hemisphere points downward at some angle. A dipping needle is a compass turned on its side so that its axis of rotation is horizontal. Its plane of rotation is placed in a N-S direction and the angle the needle takes with the horizontal is the called the magnetic inclination. The horizontal angular distance from the north to the direction of a compass is known as the magnetic declination.
Reference: Central Scientific Catalogue F, 1923, p.175; Robert Bud and Deborah Jean Warner, Instruments of Science: An Historical Encyclopedia, New York, 1998, pp.175-77.