File:Variometer antenna loading coil for spark transmitter 1920.jpg

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Description	English: An adjustable antenna loading inductor for a naval spark-gap transmitter an obsolete type of radio transmitter used during the wireless telegraphy era from about 1900-1920. It was used to tune the large wire antenna suspended from the mast to resonance with the transmitter. It consists of three spiral coils made of copper strip, with the center one suspended on hinges so it can be moved closer to one or the other of the two outside coils. The three coils are connected in series. One of the outside coils is wound in the same direction as the center coil, so when the center coil is moved closer to it the magnetic fields of the coils are in the same direction and add, increasing the inductance. The other outside coil is wound in the opposite direction to the center coil, so when the center coil is moved closer to it the magnetic fields of the two coils oppose each other and the inductance decreases.
Date	1921
Source	Retrieved December 5, 2008 from Bernard John Leggett (1921) Wireless Telegraph,: with Special Reference to the Quenched-Spark System, E. P. Dutton and Co., New York, fig. 32b facing p. 88 on Google Books
Author	Bernard John Leggett

Public domain	This media file is in the public domain in the United States. This applies to U.S. works where the copyright has expired, often because its first publication occurred prior to January 1, 1929, and if not then due to lack of notice or renewal. See this page for further explanation.	United States
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