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Golding Bird's original sketch of his interrupter circuit.
Description: The prongs at the end of the pivoted arm dip into mercury filled recesses. This completes a circuit which energises a coil around the iron pivot arm and functions as an electromagnet. The magnetic polarity is so arranged that a permanent magnet underneath the arm then repels the pivot arm and causes the circuit to break, but the prongs at the other end of the pivot arm then close an identical circuit at that end and the procedure repeats endlessly. The output of the interrupter is fed to an induction coil which greatly increases the voltage applied to the patient by transformer action.[1]

An interrupter in electrical engineering is a device used to interrupt the flow of a steady direct current for the purpose of converting a steady electric field into a changing one. Frequently, the interrupter is used in conjunction with a coil to produce increased voltages either by a back emf effect or through transformer action in an induction coil.

Medical use[edit]

Bird's interrupter[edit]

The physician Golding Bird designed his own interrupter circuit for delivering shocks to patients from a voltaic cell through an induction coil. Previously, the interrupter had been a mechanical device requiring the physician to manually turn a cog wheel, or else employ an assistant to do this. Bird wished to free his hands to better apply the electricity to the required part of the patient. His interrupter worked automatically by magnetic induction and achieved a switching rate of around 5 Hz (five times per second).[1] The faster the interrupter switches, the more frequently an electric shock is delivered to the patient and the aim is to make this as high as possible.[2]

Page's interrupter[edit]

A rather more cumbersome interrupter was constructed by the American Charles Page slightly earlier in 1838 but Bird's work was entirely independent. Although there is little in common between the two interrupter designs, Page takes the credit for being the first to use permanent magnets in an automatic interrupter circuit. Bird's (and Page's) interrupter had the medically disadvantageous feature that current was supplied in opposite directions during the make and break operations, although the current was substantially less during the make operation than the break (current is only supplied at all while the switch is dynamically changing). Treatment often required that current was supplied in one specified direction only.

Letheby's interrupter[edit]

A modified version of the interrupter was produced by Henry Letheby which could output only the make, or only the break currents by a mechanism consisting of two spoked wheels. Bird also produced a uni-directional interrupter using a mechanism we would now call split-rings. The date of Bird's design is uncertain but may predate Letheby's. Both designs suffered from the disadvantage that automatic operation was lost and the interrupter had to once again be hand-cranked. Nevertheless, this arrangement remained a cheaper option than electromagnetic generators for some time.[1][3]

Other designs[edit]

Other early interrupters worked by clockwork mechanisms or (non-magnetic) reed switches operated by motion of the patient's limbs. One example of such a device is found in the Pulvermacher chain.[4]


  1. ^ a b c Bird (1838), pp. 18–22
  2. ^ Coley, p.368
    Morus, pp. 250–251
  3. ^ Morus, pp. 250–251
    Bird, Lectures, pp. 119–122
    Letheby, pp. 858–859
  4. ^ Lardner, p.289
    Pulvermacher, p.2