LAWS. — COEFFICIENTS OF SELF INDUCTION. 263 



fastened to composition collector rings, which are carried and insulated 

 by blocks of vulcanite mounted on the tips of the teeth of one of the 

 castings. The brushes of spring copper are supported from the base 

 plate. The eight teeth were milled from the round by a special 

 Brown and Sharpe cutter, and have the form of a sinusoid laid off 

 radially from the base circle. Of course, owing to the spreading of 

 the lines of force, the particular form to be given to the teeth in order 

 to obtain the best action can only be ascertained by trial. 



The armature consists of a vulcanite disk, 10£" diameter, \" thick, 

 having a 4" hole in the middle. From this hole radiate sixteen equally 

 spaced saw-cuts -fa" deep, ^" wide, and 2\" long, the outer extremi- 

 ties of alternate pairs of cuts being connected by grooves concentric 

 with the disk. In these cuts and grooves the armature wires are laid 

 in shellac. The windings at present consist of three sections having 

 respectively 10, 10, and 20 turns of No. 30 B. & S. copper wire. The 

 total resistance. is about ten ohms. The manner of winding as well as 

 the action of the dynamo, by varying the length of active wire, are evi- 

 dent from Figure 3. The armatures are carried by built up hard-wood 

 rings, \0\" outside diameter, S\" inside, and Ifa" deep. The ring in 

 one dynamo is attached to a brass disk and hub, which fit the shaft 

 and are rigidly bolted to the frame of the machine. 



In the other dynamo, the armature, ring, disk, and hub are attached 

 to a worm-wheel, and the whole is movable on a sleeve supported 

 firmly from the frame of the machine. Through this sleeve passes the 

 shaft without contact. The worm-wheel gears into a tangent screw 

 provided with a divided head, the gearing being such that 1° on the 

 wave form is represented by 20° on the head. Backlash is prevented 

 by a spiral spring. See Figures 2 and 3. 



Between the two dynamos is a brass disk, keyed to the shaft and 

 provided with an insulated collector ring and contact point. By the 

 side of this disk is a radial arm, revolving freely about the shaft, and 

 capable of being clamped at any desired point to a fixed graduated 

 circle. The arm carries the contact spring, made of watch spring and 

 supported on its lower side very near its extremity to prevent the 

 lengthening of the arc of contact by vibration. The spring can be 

 moved out of contact with the point by a fine-pitched screw. This 

 contact arrangement we have found to work satisfactorily at as 

 many as 3,000 revolutions per minute. 



The shaft of the machine is of steel, 1" in diameter, 29" long, and 

 runs in composition bearings, each of which is 5£" long. The frame 

 of the machine is heavy, weighing 175 pounds, and is designed so 



