| An induction coil is a step-up transformer, and so requires
an alternating current input to the primary. In the nineteenth century
almost all power supplies were direct current, and so a way had to be found
to convert DC to AC. The standard technique was a mechanical interrupter
or Electrotome, run at a frequency controlled by the mechanical attributes
of the device.
The Electrotome at the left is in the apparatus collection of the University of Toronto and was made by Ruhmkorff of Paris, whose name is often associated with the Induction Coil. This is a feedback device: the current to be interrupted passes through the electromagnet on the right-hand side. This tilts the pivoted horizontal beam, and breaks the contact between the downward-pointing pin at the left end of the beam and a (missing) pool of mercury in the glass cup. The current through the coil stops, and the beam tilts counterclockwise to re-establish the mercury contact and the current through the coil. The position of the sliding spherical weight at the top controls the frequency.
| Another Ruhmkorff electrotome is in the Garland
Collection of Classical Physics Apparatus at Vanderbilt University. This
was probably purchased in the mid-to late-1870s, when the Vanderbilt apparatus
collection was being established.
Note that both mercury cups are made of glass, suggesting that the metal cup in the Toronto example is a replacement. The sliding spherical weight and its support rod are missing in the instrument at the right.
| Nicholas Callan (1799-1865 was the professor of Natural Philosophy
at St. Patrick's College in Maynooth, about twenty five miles west of Dublin,
Ireland, from 1826 to his death. He developed the first Induction
Coil in 1836, based on his large Electromagnet.
At the left is the device which Callan built to make and break the primary circuit of the coil. He called it a Repeater, a usage not followed later in the century.
| This interrupter was made by Morris E. Leeds
& Co of Philadelphia, "Manufacturers of High Grade Electrical Measuring
Instruments, X-Ray Equipment and other Scientific Apparatus". The date
is no later than 1900-1902.
It is on display in the museum room at Washington and Lee University in Lexington, Virginia.
| The electrotome at the left, in the collection of the University
of Tornonto, was made by Rudolph Koenig of Paris, and could be used to
drive an early Fourier
synthesizer, Helmholtz's large apparatus for compounding timbres of
This electrotome is in the Millington/Barnard Collection at the University of Mississippi. It carries the label of James W. Queen of Philadelphia, and was probably imported by the firm.
The date is probably 1880-1900, and it was used as an external interruptor for an induction coil.
|| The apparatus at the left, on display in the University
Museum at the University of Mississippi was made by the firm of E.S. Ritchie
of Boston, is listed as "electromagnetic apparatus." It is not in the 1861m
1869 0r 1889 Ritchie catalogues, and my guess is that it is an electrotome.