The switchboard is constructed of some fireproof material, preferably slate or marble. When the cost of this material is an item to consider, build a substantial wooden frame for your switchboard. You can then screw asbestos shingles to this to hold the various instruments and with a little care such a switchboard can be made to look business-like, and it is fully as serviceable as the more expensive kind. The switchboard instruments have already been described briefly. They consist of a voltmeter (to measure voltage); an ammeter (to measure the strength of the current drawn, in amperes), a rheostat (to regulate the voltage of the machine to suit the individual requirements); and the usual switches and fuses. The main switch should be so wired that when open it will throw all the current off the line, but still leave the field coils, the voltmeter, and the switchboard lamp in circuit. The main-switch fuses should have a capacity about 50 per cent in excess of the full load of the dynamo. If the machine is rated for 50 amperes, 75-ampere fuses should be installed. This permits throwing on an overload in an emergency; and at the same time guards against a short circuit. If the capacity of the machine is under 30 amperes, plug fuses, costing 3 cents each, can be used. If it is above this capacity, cartridge fuses, costing a little more, are required. A supply of these fuses should be kept handy at all times. _Governors and Voltage Regulators_ [Illustration: A centrifugal governor (Courtesy of the C. P. Bradway Company, West Stafford, Conn.)] The necessity for water wheel governors will vary with conditions. As a general rule, it may be said that reaction turbines working under a low head with a large quantity of water do not require as much governing as the impulse wheel, working under high heads with small quantities of water. When governing is necessary at all, it is because the prime mover varies in speed from no load to full load. Planning one's plant with a liberal allowance of power--two water horsepower to one electrical horsepower is liberal--reduces the necessity of governors to a minimum. As an instance of this, the plant described in some detail in Chapters One and Six of this volume, runs without a governor. However, a surplus of water-power is not usual. Generally plants are designed within narrow limits; and then the need of a governor becomes immediately apparent. There are many designs of governors on the market, the cheapest being of the centrifugal type, in which a pair of whirling balls are connected to the water wheel gate by means of gears, and open or close the gate as the speed lowers or rises. Constant speed is necessary because voltage is directly dependent on speed. If the speed falls 25 per cent, the voltage falls likewise; and a plant with the voltage varying between such limits would be a constant source of annoyance, as well as expense for burned-out lamps.