There are two main designs of this type of break, one with  a perpendicular dipper and the other rotary, each being  driven by a small motor on a circuit independent of the circuit  of supply to the coil.

The perpendicular dipper is worked by a crank motion  attached directly to the shaft of the motor. It allows of simple  and exact regulation of the amount of dip and consequent  duration of contact, as well as of the rate of speed. This is a  simple and straightforward mechanism, and does not readily  get out of order. It is also easily cleaned, so that an instru-  ment-maker need seldom, if ever, be called in to assist in its  continued working. A high rate of speed cannot be attained  —about 1,000 to 1,500, and the break is somewhat noisy in  action, but for combined work it acts on the whole very well.  The rotary design of dipper break, usually associated with  the name of Mackenzie Davidson, has an inclined axle, on  which the dipping blade or blades are fixed radially. Those  radial blades make and break contact with the mercury while  the axle revolves. With this break a higher rate of speed can  be attained than with the perpendicular dipper, but the inter-  vention of a belt drive, as in the usual form, is a disadvantage;  while the mechanism is less simple, and may require skilled  assistance more frequently to keep right. More recent forms  have the motor set on an inclined base, and the vanes attached  to a direct prolongation of its shaft, as shown in Fig. 31.  Either of these dipper breaks serve very well for moderate  currents and for medium rates of interruption.

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