=454. Directions.= (A) Arrange as in Fig. 149. Place the compass a short distance from the end of the core of the coil H (No. 89). Close the circuit, and as soon as the needle gets part way around open it again, closing it at the proper time to give the needle a new impulse. The speed can be regulated, somewhat, by changing its distance from the core. A key may be used in place of a reverser. (B) Reverse the direction of rotation. _=455. Discussion of Exps. 189-190.=_ We have, in these experiments, the key to the action of electric motors. By properly opening and closing the circuit, the rotary motion can be kept up as long as current is supplied. If a small pulley were attached to the top of the compass-needle in Exp. 190, a tiny belt could be attached, and we should have a machine that could do, perhaps, a fly-power of work. (See Electric Motors.) CHAPTER XXVII. APPLICATIONS OF ELECTRICITY. _=456. Things Electricity Can Do.=_ Among the almost countless things that electricity can do are the following: It signals without wires. It drills rock, coal, and teeth. It cures diseases and kills criminals. It protects, heats, and ventilates houses. It photographs the bones of the human body. It rings church bells and plays church organs. It lights streets, cars, boats, mines, houses, etc. It pumps water, cooks food, and fans you while eating. It runs all sorts of machinery, elevators, cars, boats, and wagons. It sends messages with the telegraph, telephone, and search-light. It cuts cloth, irons clothes, washes dishes, blackens boots, welds metals, prints books, etc., etc. As this book deals almost exclusively with experiments, to be performed with simple, home-made apparatus, space cannot be given for a discussion of the many instruments and machines which make electricity a practical every-day thing. (See "Things A Boy Should Know About Electricity.") The principles upon which a few important instruments depend, however, will be given. [Illustration: Fig. 150.] =EXPERIMENT 191. To study the action of a simple "telegraph sounder."= =457. Directions.= (A) Arrange as in Fig. 150. The electromagnet is supported upon its base, as directed in § 407. Coil H, K, and D C are joined in series. The iron strip, I, can be held by the left hand, while K is worked with the right. (B) Press the key, closing the circuit for different lengths of time, and note that the _armature_, I, responds exactly to the motions at K. _=458. Discussion.=_ The downward click makes a distinct sound, and in regular instruments the armature is allowed to make an upward click, also. The time between the two clicks can be short or long to represent _dots_ or _dashes_, which, together with _spaces_, represent letters. (For telegraph alphabet, and complete directions for making and connecting a home-made telegraph line, see Apparatus Book.) [Illustration: Fig. 151.]