The dynamo-electric machine should not be driven without its proper leading wires to lamp and lamp being connected up, or at least an external resistance equivalent to that of the lamp (which is approximately one Siemens' unit) must be inserted. In other words, the machine must not be driven when a wire of small resistance connects the two terminals C and Z. This is expressed more briefly by saying the machine must not be short-circuited. If it is short-circuited when in motion the electric current becomes so powerful that it will leap from segment to segment of the commutator, where very bright and large sparks will be seen, and if continued would destroy the insulation, thus weakening the current generated.
The leading wires should never be disconnected suddenly while the machine is revolving at its full speed, as such a sudden interruption will produce an intense spark, which will burn the ends of the wire where the contact is suddenly broken. When it becomes necessary to disconnect the wires, the belt should be pushed on to the loose pulley by means of the striking gear, or the steam engine should be stopped.
It may be here stated that all connections should be cleaned bright and screwed tightly, to ensure perfect metallic contacts being made.
Coupling two Machines.—At [Fig. 174] is shown a diagram of how to make the connections when coupling two machines in parallel circuit. MM', m, m', represent the ends of the wires of the electro-magnets; BB' are the branches; C and Z are the terminals of each machine respectively.
The three ways in which the various wire connections of these machines are joined up, and which are enough for all ordinary purposes, are given below in paragraphs (a), (b), and (c).
(a) When the machine is working singly and revolving in the direction indicated in [Fig. 166], the following connections are made:—
| M | is connected with | B, |
| M' | " | B', |
| m | " | Z, |
| m' | " | C, |