*The Brushes* are made of spring copper according to the shape and dimensions shown in Figure 141. They can be cut out with a pair of snips. Each brush is mounted on a small fibre block supported on the large motor bearing. The holes marked A and C in the illustration should be threaded with a 4-36 tap. The hole B should be made one-eighth of an inch in diameter and drilled all the way through the block. The holes A and C are used to fasten the blocks to the bearing. The brushes are fastened to the blocks by means of a 6-32 screw with a nut on the lower end. *The Base* is a rectangular block, three inches wide, three and one-half inches long and three-eighths of an inch thick. The motor is fastened to the base by four small right angled brackets bent out of strip brass and secured to the field frame by two machine screws passing through the holes H and H into a nut at the opposite end. [Illustration: FIG. 141.—The Brushes.] One terminal of the field winding is connected to a binding post mounted on the base. The other terminal of the field is connected to the right hand brush. The end of the wire should be placed under the head of the screw which holds the brush to the fibre block. The brush should be on the under side of the block so that it bears against the under side of the commutator. The left hand brush bears against the upper side of the commutator and is connected to a second binding post on the base of the motor. This makes it a "series" motor, that is, the armature and the field are connected in series. [Illustration: FIG. 142.—The Fibre Block for supporting each Brush.] The motor is now ready to run. Put a drop of oil on each bearing and make certain that the curved portion of the brushes bear firmly against the centre of the commutator on opposite sides. The armature having three poles, should start without assistance and run at high speed as soon as the current is applied. Two cells of dry or other battery should be sufficient. The motor may be fitted with a small pulley so that its power may be utilized for driving small models. CHAPTER XIV. HOW TO BUILD AN ELECTRIC ENGINE. An Electric engine is really a form of electric motor but differs from the most common form of the latter in that the armature, instead of revolving, oscillates back and forth, like the piston of a steam or gasoline engine. Electric engines are not as efficient as electric motors from the standpoint of the amount of power delivered in proportion to the current used, but they make very interesting models and the young experimenter will derive fully as much pleasure in constructing one as from the construction of an electric motor. Various forms of electric engines were made before the first practical electric motor was invented. They amounted to little more than curiosities, however, and could only be used where the expense of electric current was not to be regarded. [Illustration: FIG. 143.—Completed Electric Engine.]