Although the circuit, when the machine is in full action, should never be suddenly interrupted, interruption arising from the extinction of the light is _not_ dangerous, because it is always preceded by a decrease in the strength of the current. When it is desired to divert the current into another circuit it is advisable to stop the machine. Although in practice with small machines this is rarely done, with large machines it is necessary. _Self-acting Shunt._--For great security, especially with the two machines coupled together, where the electric current is strong and the light equivalent to about 14,000 candles, it is advisable to insert in the circuit a self-acting shunt. [Illustration: Fig. 167.] This is placed between the lamp and machine and connected to both leading wires. Its principle is as follows:-- The terminal _M_, Fig. 167, is joined by a short connecting wire to one terminal of the machine. The terminal _L M_ is connected to the remaining terminal of the machine and also to one of the lamp terminals. The terminal _L_ is connected to the other terminal of the lamp. The shunt contains a small electro-magnet _E_ mounted upon a square wooden slab or baseboard with its armature a, a contact c, and, below the slab, a resistance coil _W_, which is equal to the resistance of the electric arc of the light, about 1 S. _u._[W] As long as the lamp is burning well, the current circulates in the coils of the electro-magnet, and the armature _a_ being strongly attracted, there is no contact at _c_. The resistance coil _W_ is therefore not in electrical circuit. When the light is extinguished the current in the coils of the electro-magnet ceases, and the armature is withdrawn by the spring _f_ making contact at _c_. This offers to the electric current a path through _W_ of equal resistance to that of the lamp, and the current is subjected to scarcely any change, so that the motor has practically no cause to alter its rate. When the carbon points of the lamp again touch, the electric current returns to them, breaking contact at _c_, re-establishing the former conditions. _Direction of Rotation._--The armature may revolve in either direction. If it becomes necessary to drive it in the opposite direction to that for which the machine has been made, it is only necessary to reverse the brushes, placing their points in the direction of motion, and to change two of the wire connections, which operations can be effected in a few minutes. Fig. 166 shows the position of brushes for one direction of rotation and Fig. 168 that for the other. [Illustration: Fig. 168.] _Conducting or Leading Wires._--The leading wires are usually of copper of high electrical conductivity. They must be insulated from one another the whole of their length and not placed too close together. As their resistance affects the intensity of the light very much, the section must be carefully proportioned to the distance of the lamp from the machine.