Double Helix With Interrupter
   Double Helix is Page's name for a step-up transformer. "It consists of a double helix into which a bundle of iron wires can be inserted. The inner helix [the primary] is wound of two or more layers of coarse insulated copper wire... The outer helix [the secondary] is completely insulated from the other, and consists of fine insulated copper or iron wire." -- from the 1842 Manual of Magnetism.
   The primary is connected to the electro-chemical cell through the rasp or serrated brass bar mounted along the top of the instrument. Moving the loose wire along the top of the rasp makes and breaks the contact, producing a pulsating alternating current signal in the primary, which then produces a high-voltage signal in the secondary 

   In the cut the output of the secondary is connected to a couple of metallic handles for a use in medical electricity. There is no need to throw a switch to start the apparatus; there is an output only when the rasp is activated. 

   Davis used also used this coil with an external interrupter for medical purposes.

   The double helix below was given to the Wellesley College physics department in 1937 by a physician in honor of his daughter who graduated that year. It is now in the Greenslade Collection. The apparatus is not listed in the 1851 edition of Davis's Manual of Magnetism, but has all of the Davis features, including the strongly-grained wooden base, the wide straps holding down the coil and the flattened ball feet. The interrupter is an electromagnet pulling on a vibrating reed. My assumption is that this was made by a Davis successor, such as Palmer and Hall, after Davis sold his firm in the early eighteen fifties.

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