A very ingenious application of the dynamo is shown at Figs. 16 and 17. The electric generator is built in such a manner that it forms an integral part of the power plant. The magneto field is produced by a series of revolving magnets which are joined to and turn with the fly wheel of the motor. The armature coils are carried by a fixed plate which is attached to the engine base. This apparatus is really a magneto having a revolving field and a fixed armature, and as the magnets are driven from the fly wheel there is no driving connection to get out of order and cause trouble.

As the coils in which the current is generated are stationary, no commutator or brushes are needed to collect the current because the electricity may be easily taken from the fixed coils by direct connection. It has been advanced that this form of magneto is not as efficient as the conventional patterns, because more metal and wire are needed to produce the current required. As the magnets which form the heavier portion of the apparatus are joined to the fly wheel, which can be correspondingly lighter, this disadvantage is not one that can be considered seriously because the magnet weight is added to that of the motor fly wheel, the

combined weight of the two being that of an ordinary balance member used on any other engine of equal power.

Methods of Winding Dynamos.-The reader not versed in electrical science is apt to be puzzled by the designation of the various windings used on dynamos and motors. The armature windings and field coils may be connected together in a number of ways, as outlined at Fig. 18. The simple machine shown at A uses a permanent magnet to produce the field and therefore has only one set of windings to be considered, i. e., those on the armature. When the field magnet is an electro magnet another set of windings must be considered, i. e., those of the field magnet. When the current generated in the armature must first pass through the field windings before it reaches the external circuit the machine is said to be a series wound machine as shown at B because the armature and field windings are joined together in series.. If only a portion of the current generated by the armature is directed to the field magnet windings the machine is said to be shunt wound, as shown at C. A compound wound dynamo is shown at D. In this two sets of field windings are used, one connected in shunt, the other coils in series. The shunt winding provides an initial excitation sufficient to generate full voltage at no load. The series coils provide an excitation that increases as the load increases and thereby strengthen the field so as to prevent the falling off in voltage that would otherwise occur. If the series coils are sufficiently powerful to make the voltage rise as the load increases the machine is said to be over-compounded.

The compound wound dynamo is the type used almost universally for direct current production. In stationary applications, compound wound motors are used where the load varies considerably under which conditions the extreme speed variation of series motors would be objectionable and where increased torque or turning power would be needed that shunt motors could not give. A compound wound dynamo is, to a certain extent, self-regulating, as the two coils counteract each other and bring about a more regular action for varying currents than that of the ordinary shunt or series wound dynamo. The extent of the regulation possible depends upon the proportions of the different windings though a compound

Fig. 18.-Diagram Showing Methods of Winding Dynamo. A-Simple Magneto Generator. B-Series Wound Machine. C-Shunt Wound Machine. D-Compound Typc.

wound machine can be self-regulating at only one particular rotative speed. In a series wound dynamo short circuiting or lowering the resistance of the external circuit strengthens the field, thereby increasing the electro-motive force and the current strength. Some cut out means are usually provided to break the external circuit or to interpose added resistance to keep the current strength relatively constant and prevent injury to the windings by heating of the wire and melting of the insulation. In a shunt wound dynamo the lowering of resistance on the outer circuit takes current from the field and lowers the electro motive force of the machine. Short circuiting has no heating effects. A compound wound machine combines, to a certain degree, the features of both the shunt and series wound dynamo. In a dynamo where the armature windings are grouped in coils which have independent terminals and which are not connected in series, the construction is termed "open coil." The terminals are attached to separate divisions of the commutator and are so spaced that the collecting brushes touch each pair belonging to the same coil simultaneously. The brushes therefore take current from only one coil at a time. In a closed coil dynamo, the armature windings are connected in series and current is delivered from all coils.

Electrical Terms Defined.-In referring to any force it is necessary to have some units by which its capacity may be judged. For instance, in comparing bodies of different size we can use units which will show the difference of mass or dimensions, such as pounds or feet, or the fractions and multiples thereof. To gauge the ability of the electric force there are several practical units with which all motorists should be familiar. They are the volt, watt, ohm and ampere.

The VOLT is the practical unit of electro-motive force, pressure, or difference of potential or condition, existing between different parts of the circuit. Referring again to the reservoirs of water; we would find a foot height of liquid a very convenient expression to use as a difference of height or head of water, and such is in constant use by all engineers. This is a precise analogy to the volt which is the unit that measures the tendency of an electric charge to escape to the opposite level, this being the actuating force.

of currents. The volt is the cause of the current, not a part of it. It is the difference in voltage which causes a current to flow from one object to another. The expression 100 or any other number of volts current has no foundation; the expression should be, 100

volt circuit. The familiar dry cell maintains a difference of about one and one-quarter volts between the surfaces of the elements of which it is composed.

The OHM is the unit by which resistance is judged. Everything has electrical resistance. Some elements have very little,