With this type of break at rest, there is no danger of the  continuous current being accidentally passed direct to the  coil, since the jet by which the contact is made is not formed  until some rate of speed is got up.

A higher rate of interruption may be obtained with such^a  break than with a dipper; but for lower rates of interruption  those breaks are not good. For a time the action at a suitable  rate is very satisfactory; but the break is somewhat easily  put out of order, and requires frequent cleaning, which is a  difficult and dirty process. More recent forms are simpler in  construction and more easy to clean when clogged.

The earlier forms (as in Fig. 32) are driven by belt-con-  nection from the motor, and the slipping of this may cause  much inconvenience. More recent jet breaks are driven by  some electro-magnetic arrangement directly connected to the  rotating parts.  Thus, in Gaiffe’s form (as shewn in Fig. 33), four electromagnets are situated on the cover of the instrument, and  through the coils of these the current passes before entering  the interrupting mechanism on its way to the primary of the  induction-coil. This is a doubtful advantage, since the rate  of interruption cannot be varied independent of the strength  of current supplied to the coil, or vice versa; whereas that is  a latitude desirable under certain conditions. With a heavy  load the action is somewhat noisy ; but this is on the whole a  reliable instrument, and has the further advantage of being  less than half the price of the older forms.

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