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.