Dynamos may be connected to water wheels either by means of a belt, or the armature may spin on the same shaft as the water wheel itself. The latter is by far the more desirable way, as it eliminates the loss of power through shafting and belting, and does away altogether with the belts themselves as a source of trouble. An installation with the water wheel and armature on the same shaft is called a "direct-connected set" and is of almost universal use in large power plants. To be able to use such a direct-connected set, the dynamo must be designed to develop its full voltage when run at a speed identical with that of the water wheel. That is, if the dynamo is wound to be run at a speed of 800 revolutions per minute, it must be driven by a water wheel which runs at this speed and can be governed within narrow limits. Small impulse wheels running under great heads attain high speed, and for such wheels it is possible to obtain a suitable dynamo at low cost. For instance, a 12-inch impulse wheel, running under a 200-foot head will develop 6-3/4 horsepower when running at a speed of 875 revolutions per minute. A dynamo for direct coupling to such a wheel should have a rated speed within 5 per cent of 875 r.p.m.; and, as generators of this speed are to be had from the stock of almost all manufacturers, there would be no extra charge. When it comes to the larger wheels, however, of the impulse type, or to turbines operating under their usual head the question becomes a little more difficult. In such cases, the speed of the water wheel will vary from 150 revolutions per minute, to 400, which is slow speed for a small dynamo. As a general rule, the higher the speed of a dynamo, the lower the cost; because, to lower the speed for a given voltage, it is necessary either to increase the number of conductors on the armature, or to increase the number of field coils, or both. That means a larger machine, and a corresponding increase in cost. In practice, in large plants, with alternating-current machines it has become usual to mount the field magnets on the shaft, and build the armature as a stationary ring in whose air space the field coils revolve. This simplifies the construction of slow-speed, large-output dynamos. Such a machine, however, is not to be had for the modest isolated plant of the farmer with his small water-power. [Illustration: Instantaneous photograph of high-pressure water jet being quenched by buckets of a tangential wheel] [Illustration: A tangential wheel, and a dynamo keyed to the same shaft--the ideal method for generating electricity. The centrifugal governor is included on the same base]