Induction Coil #10226
Unsigned
An emf (or voltage) is induced in a coil by changing the magnetic field in the coil. By making the field change rapidly and using a large number of turns in the coil, a very high voltage can be produced. There is an interrupter in the circuit of the primary coil to produce the rapidly changing magnetic field and the high voltage in the secondary coil. Induction coils were first used for medical purposes but nowadays they provide the high potentials needed to operate spark plugs in automobiles and gaseous discharge tubes such as neon tubes for signs.
References: Gerard L’E Turner, The Practice of Science in the Nineteenth Century, Haarlem, the Netherlands, 1996, pp. 243-46; James W. Queen & Co. Catalogue I-66 Electrical Testing Apparatus, 1887, p.107-10; Max Kohl Catalogue No. 100 (c.1927) pp. 979-84; Robert Bud and Deborah Jean Warner, Instruments of Science: An Historical Encyclopedia, New York, 1998, pp.328-30.

Weinhold’s Apparatus #10106
Unsigned
By turning the crank, one can convert direct current into low-frequency alternating current. The field varies in such a way that the compass needle rotates in accordance with the position of the crank. Other plug-in coils may be used to demonstrate inductance. At a time when AC was beginning to replace DC in electrical distribution systems, this apparatus helped illustrate the relationship of the two to students. D.B. Brace held a patent for a distribution system that could use either AC or DC.
References: Max Kohl Catalogue No. 100 (c.1927) p.998; D.B. Brace, Patent No. 484,549 "System of Electrical Distribution," 1892.

Ayrton and Perry Variable Inductor #10090
Leeds & Northrup, Philadelphia
Two coils of slightly different sizes are connected in series and wound on spherical surfaces with the smaller rotatable inside the larger. A dial at the top indicates the inductance. When the coils are aligned such that their currents are in opposite directions, the inductance is nearly zero. When at 90^o the inductance is the sum of the self-inductances of the two coils. When aligned with their currents in the same direction it is the sum of the self-inductances plus twice their mutual inductance.
References: Frank A. Laws, Electrical Measurements, New York, 1938, pp.352-53; Whipple Museum Catalogue 8: Electrical and Magnetic Instruments, 1991, No. 389.

Inductive Repulsion Apparatus #10178
Max Kohl, Chemnitz
This apparatus, designed by Elihu Thomson, consists of a magnetizing coil and a sliding core, which is a bundle of iron wires. An alternating current of 12-16 amperes is applied to the coil. The device is used in a number of demonstrations of electromagnetic induction. One example is the "jumping ring" in which an aluminum ring is placed around the core. When the current in the coil is switched on, a current is induced in the ring. The magnetic field due to this induced current interacts with that in the core producing a repulsion making the ring fly high in the air.
References: Max Kohl Catalogue No. 100 (c.1927) p.1006; Richard Sutton, Demonstration Experiments in Physics, New York, 1938, pp.349-50.