A sealed glass tube pivoted near its centre contains a portion of glycerine sufficient to considerably overbalance it and keep one end down. A soft iron armature is attached to this tube in such manner that each time a current flows through a pair of electro-magnets, the attraction of the armature causes the tube to tilt and the glycerine flows along to the other end. Now it will be readily seen that if the tube is only tilted for a second or so, the slow-moving glycerine will not have flowed sufficiently to the end to overbalance it, but it requires an attraction of the armature for a considerable period. This electro-magnet is in circuit with the gas-lighting wires, the tube being provided with contacts in such manner that, when fully tilted, the circuit is broken. The momentary jerks imparted to the armature by the operation of a pendant or automatic burner will not be enough to permanently tilt the tube and break contact, but a short circuit will hold the armature tight down, until the increasing weight of glycerine causes the tube to open the circuit.
Fig. 28.
Fig. 29.
Automatic Sectional Cut-out.
Fig. 30.
This cut-out, Fig. 29, is representative of the class which use clockwork, and is both simple and reliable. The house circuit is in series with an electro-magnet which controls a clockwork having a long pinion shaft. This clockwork starts and runs while the house circuit is closed, as on operating a burner, but stops when the circuit is opened and flow of current ceases. The wires leading to different circuits in the building run through a number of contact springs mounted on sliding rods, which have teeth cut on the under side (Fig. 30). These rods have soft iron armatures on the opposite ends from the contact springs, which rest over electro-magnets, also connected to the house circuits. When the clockwork starts, the pinion shaft revolves, but does not engage in any of the sliding rods, as they just clear it. Should a heavy or continuous current pass through one of the electro-magnets, it attracts the armature on the corresponding rod (Fig. 31), and the turning pinion engages in the teeth, drawing up the rod and breaking contact.