For the proper control of the alternating current in any of the numerous systems described in the previous chapter, various devices, which might be classed as "auxiliary apparatus," are required. These may be grouped into several divisions, according to the nature of the duty which they perform, as

1. Switching devices;

• a. Ordinary switches;
• b. Oil break switches;
• c. Remote control switches.

2. Current or pressure limiting devices;

• a. Fuses;
• b. Reactances;
• c. Circuit breakers;
• d. Relays.

3. Lightning protection devices;

• a. Air gap arresters;
• b. Multi-gap arresters;
• c. Horn gap arresters;
• d. Electrolytic arresters;
• e. Vacuum tube arresters;
• f. Choke coils;
• g. "Static" interrupters.

4. Regulating devices;

• a. Induction voltage regulators;
• b. Variable ratio transformer regulators { drum type;
• { dial type;
• c. Compensation shunts;
• d. Pole type regulators;
• e. Small feeder voltage regulators;
• f. Automatic voltage regulators;
• g. Line drop compensators;
• h. Starting compensators;
• i. Star delta switches.

5. Power factor regulating devices;

• a. Condensers;
• b. Synchronous condensers.

6. Indicating devices;

• { plunger type;
• a. Moving iron instruments { inclined coil type;
• { magnetic vane type;
• b. Hot wire instruments;
• c. Induction instruments { shielded pole type;
• { repulsion type;
• d. Dynamometers;
• e. Instrument transformers;
• { commutator type;
• f. Watthour meters { induction type;
• { Faraday disc type;
• { synchronous motor type;
• g. Frequency indicators { resonance type;
• { induction type;
• { lamp type;
• h. Synchronism indicators { voltmeter type;
• { resonance type;
• { rotating field type;
• i. Power factor indicators { wattmeter type;
• { rotating field type;
• j. Ground detectors;
• k. Earth leakage cut outs;
• l. Oscillographs.