The sources from which power may be drawn on the plane are four, although the various combinations of these present a large number of alternative approaches to the problem. These sources are:

1. The engine of the plane.

2. Wind motors.

3. Spring motors.

4. Electric motors.

These may first be considered largely from the descriptive standpoint, leaving questions of performance and efficiency for separate treatment.

Power may be derived directly from the engine through a flexible shaft, similar to that used for the revolution counter. This source of power is inherently the most direct and efficient, since the engine is the seat of all the lifting and driving energy of the plane. There is no loss through transformation into other forms of energy, such as electrical; or by the use of more or less inefficient intermediary apparatus, such as wind propellers. Against the direct drive of the camera from the engine may, however, be urged that the usual distance between engine and camera is too great for reliable mechanical connection, as by flexible shafting. Objections also arise from the standpoint of speed. This cannot be controlled by the camera operator; and varies over too wide a range, as the engine changes from idling to full speed, to fit it for automatic camera operation. The first objection may be met by that combination of methods of power drive which consists in transmitting the power electrically; that is, by letting the engine operate a generator from which cables run to a motor close to the camera. This method, of course, sacrifices efficiency, and it breaks down when the engine speed drops below the speed necessary to generate the requisite voltage. This defect may in turn be met by floating in storage batteries, which brings up the whole question of electrical drive, to be treated presently. While use of the engine for direct drive or for generating electric current has not been adopted in the American service, it is known that some German planes were supplied with electric current in this way.

Coming next to the wind motors, these possess one very great merit: they utilize a motive power that is always present as long as the plane is in motion through the air.

On the other hand, the process of using the main propeller of the plane to pull another smaller propeller through the air appears a roundabout way to utilize the driving power of the airplane engine. Yet on the whole it is probable that some form of propeller or wind turbine is the simplest and most convenient device we have for the operation of airplane auxiliaries. As long as the amount of power required is small, such inefficiency as is inherent in its use is offset by its convenience and reliability. An advantage of the propeller is that its speed is almost directly proportional to that of the plane through the air, a desirable feature in automatic cameras provided the proportionality is under control. Yet it is just in this matter of varying the speed at will that the propeller presents difficulties, to be met only by additional mechanisms for gearing down or governing. Propellers have the practical disadvantage that they present an easily bent or broken projection to the body of the plane (Figs. 83 and 84). The strength of small propellers for operating auxiliaries is never so much in question with reference to their resistance to whirling and thrust of air as it is to their ability to withstand the inevitable knocks and careless handling that will fall to their lot. The propeller bracket is just what the pilot is looking for to scrape the mud off his boots before climbing in.

The wind turbine has the advantage over the propeller that its speed can be varied rather simply by exposing more or less of its face to the wind. A turbine fitted with an adjustable aperture for admitting the wind is shown in Fig. 64, in connection with the type K automatic film camera. The turbine has the advantage of being compact and lying close against the body of the plane. In the form figured, altogether too much head resistance is offered—just as much for low as for high speeds—but with proper design this need not be the case. It is, moreover, quite too small to give the needed speed regulation, as it only begins to operate near its full opening.

Spring motors have the very real advantage that by their use the camera can be made entirely self contained. The simplest application of the spring motor would be to the semiautomatic camera, where no close regulation of speed is required. In such a camera the operation of exposing the shutter would release the spring, which would then change the plate or film and re-set the shutter, repeating this operation as long as the spring retained sufficient tension. Small film hand-cameras of this type, using self-setting between-the-lens shutters, have been designed, though not for aerial work. The possibilities of using springs as motive power in semi-automatic cameras have not apparently been seriously considered.

When a spring motor is used for automatic camera operation it at once becomes necessary to add to the motor an elaborate clock mechanism for controlling and regulating its speed of action. Springs are much better fitted for giving power by quick release of their tension than by slow release, and the necessary clock mechanisms for their regulation become very heavy, as well as complicated and delicate, when they are made large enough to do any real work. For their repair they require the services of clock makers rather than the usual more available kind of mechanic.

Coming next to electric motors, we meet with a source of power of very great flexibility both in its derivation and in its application. If a source cf electric current is already provided for heating and lighting as it is on the fully equipped military plane, and if it has sufficient capacity to handle the camera, its use is rather clearly indicated, irrespective of how efficiently or by what method it is produced. Especially is this the case, from the standpoint of economy and simplicity, if a propeller-driven generator is the source of current, and the alternative power drive is an additional propeller for the camera. If, on the other hand, the camera must have its own source of electric power, the advantages and disadvantages must be closely scrutinized. In this case either a generator must be provided, or resort be made to storage batteries, or a combination of the two.

Ruling out a special propeller-driven generator, we are left with either the generator driven from the engine or the storage battery. Inasmuch as storage batteries are practically indispensable with generators, in order to maintain the voltage constant at all speeds, it is on the whole advisable to rely upon batteries alone. An advantage of their use is that the power plant is entirely within the plane: All projections such as propellers are avoided. Another merit is that the power is drawn upon only as needed. Against storage batteries is their weight, the need for frequent charging, and their loss of efficiency at low temperatures—a loss so serious with those of the Edison form as to preclude their use.

When once the source of electrical energy is decided upon, its method of application needs to be considered. Here we meet at once the peculiar merit of electrical energy, namely, the ease and convenience with which it may be transmitted. All we need is a pair of wires, led to any part of the plane by any convenient route and connected by simple binding posts. It may with equal ease be turned on or off by merely making or breaking a contact with a switch. For operating semiautomatic cameras this feature may be utilized in the interest of economy, if the power is automatically turned off as soon as the plate-changing operation is finished. Exceptionally reliable make and break contacts are necessary to insure the success of this latter scheme.

Two methods of transforming the supply of electrical energy into mechanical motion are available. The first is by the use of a solenoid and plunger. This is a device practically restricted to semi-automatic cameras, in which the operation consists of a straight to-and-fro motion, initiated at the will of the operator. It has been used little if at all. The second motion is the continuous rotary one secured by the use of an electric motor. This motion is the most practical one for the continuous operation of any mechanism, but on the other hand requires that the imposed load be reasonably uniform at all times through the cycle of operations. Assuming that the camera mechanism is of this character, the motor may be attached directly to the camera, or if it must be so large as to cause danger by vibration, it may be connected through a flexible shaft. This use of an electric motor is very practical for semi-automatic cameras such as the "L" or the American deRam, in planes supplied with a suitable source of current.

When it comes to entirely automatic cameras, where uniform and regulatable speed is required, as in making overlapping pictures for mapping, the electrical drive is not so conveneint. The shunt-wound motor runs at nearly constant speed, while the series-wound motor in which the speed can be regulated by the interposition of resistance, has nothing like a sufficient range of variation for the purpose (at least five to one is imperative) before it fails to carry the load. Hence we must either incorporate in the camera some mechanism for varying the interval between exposures while the speed of the motor remains constant, or introduce an auxiliary device to effect the required transformation in speed. If we do use an auxiliary device the train of apparatus, consisting of battery (or generator), motor, speed control and camera, is altogether too long; it is apt to cause annoying delays in connecting up in an emergency, and it offers an excessive number of chances for break-down. '