Important Topics 1. Receiver: parts, action. 2. Transmitter: parts, action. 3. Induction coil, bell, line wires, etc. 4. Action of the whole device. Exercises 1. State three important electrical laws or principles that are employed in the operation of the telephone. What is the application of each? 2. Connect the binding posts of a telephone receiver with a sensitive galvanometer and press on the diaphragm of the receiver; a deflection of the galvanometer will be noticed. Release the diaphragm and a reflection in the opposite direction is seen. Explain. 3. Is the current passing through the transmitter the one going to the receiver of the instrument? Explain. 4. Does the receiver at the telephone used by a person repeat the speech of the person? Explain. 5. How many 0.5 ampere lamps can be used with a 6 ampere fuse? 6. Why is it necessary to have a rheostat connected in series with a stereopticon or moving picture machine while a rheostat is not used with arc lights out doors? 7. How many candle power should a 60 watt carbon incandescent lamp give, if its efficiency is 3.4 watts per candle power? 8. Three incandescent lamps having resistances of 100, 150, and 240 ohms, respectively, are connected in parallel. What is their combined resistance? Review Outline: Induced Currents Induced currents; 3 laws, illustrations. Construction, action, and uses of--magneto, dynamo, induction coil, transformer, motor, telephone. Mercury arc rectifier. Terms--primary, secondary, for coils and currents, armature, commutator, slip ring, brush, rectifier, open core, series, shunt, and compound connections for dynamos. CHAPTER XV SOUND (1) SOUND AND WAVE MOTION =317. What is a Sound?=--This question has two answers, which may be illustrated as follows: Suppose that an alarm clock is set so that it will strike in one week and that it is placed upon a barren rock in the Pacific Ocean by sailors who immediately sail away. If when the tapper strikes the bell at the end of the week no ear is within a hundred miles, is any sound produced? The two view-points are now made evident, for some will answer "no" others "yes." Those answering "no" hold that sound is a _sensation_ which would not be produced if no ear were at hand to be affected. Those answering "yes" understand, by the term sound, _a mode of motion capable of affecting the auditory nerves_, and that sound exists wherever such motions are present. This latter point of view is called the _physical_ and is the one we are to use in this study. [Illustration: FIG. 314.--The tuning fork is vibrating.]