(gramme) wound right-handedly over the core revolving righthandedly in a horizontal magnetic field having the N pole on the right hand.

Ans. The induced E.M.F.'s tend to make the currents climb, in both the ascending and descending halves, toward the highest point of the ring.

3. A dynamo's field magnet gives a flux of 9,000,000 lines. How many conductors must there be on the armature in order that the dynamo may generate 108 volts when driven at a speed of 600 revolutions per minute? Ans. 120.

4. You have an engine which will drive a dynamo at a fairly constant speed at all loads. How would you excite the dynamo if it were intended for lighting by incandescent lamps? Make a diagrammatic sketch of all necessary connexions, including the lamp circuit.

= €140 and

5. Take the equation E = a sin (2nt). Let a n = €100. Now take different values for t, beginning t = '0005 of a second, then t001, taking 20 different values until t = 01. Fill in the values in the above equation and find the corresponding 20 values of E. Then plot on squared paper taking E as ordinate and t as abscissæ. The result will be a curve like that shown in Fig. 251.

6. Repeat the process of the last question, taking the equation Cb sin (2nt), where b 20, n = 100, and = 0.5 radian. Plot the results upon the same paper as the curve in the last equation was plotted. One curve represents the E.M.F. at each instant, the other the lagging current.

7. An alternating pressure of 100 (virtual) volts following a sine law with a frequency of 100 per second is applied to the ends of a coil having a resistance of 8 ohms and a coefficient of self-induction of 0·005 henry; find the current that will flow and the angle of lag.

Ans. Current

=

11.6 amperes; lag 22 degrees.

8. An alternate-current magnet with properly laminated core has a coil of 160 turns, and a coefficient of self-induction of 0.005 of a henry. What alternating voltage of frequency 100 per second must be applied to it in order to obtain 4800 ampere-turns, assuming the resistance to be negligible?

Ans. 94.2.

9. How much resistance must be put in circuit with the coils of this magnet in order that the angle of lag may be 45°?

Ans. 3.14.

10. An alternate-current transformer is designed to give out 40 amperes at a pressure of 50 volts at its secondary terminals. No. of windings 300 primary; 12 secondary. Resistances 12

ohms, primary; 0014 ohm, secondary. Find the coefficient of transformation, and the volts that must be applied at the primary terminals.

Ans. Coefficient of transformation is 25; volts at pri mary terminals 1283.

11. State the principles upon which continuous-current transformers are made. Why is it necessary to have a moving part in continuous-current transformers and not in alternate-current

transformers?

12. Enumerate three distinct kinds of alternate-current motors, and state which kind is synchronous and which not.

13. An alternate-current synchronous motor is supplied from the street mains. It is found that when fully loaded it takes more current than when lightly loaded, though it always goes at the same speed and the volts remain constant. Explain how this comes about.

14. How can you produce a rotatory magnetic field? scribe some of its properties.

QUESTIONS ON CHAPTER XI

De

1. It is found that a single Daniell's cell will not electrolyze acidulated water, however big it may be made. It is found, on the other hand, that two Daniell's cells, however small, will suffice to produce continuous electrolysis of acidulated water. How do you account for this?

2. From the table of electro-chemical equivalents (Art. 240) calculate how many coulombs it will take to deposit one grain of the following metals: -Copper (from sulphate), silver, nickel, gold. Ans. Cu 3058, Ag 891, Ni 3286, Au 1473.

3. A battery of 2 Grove cells in series yiel is a current of 5 amperes for 2 hours; how much zinc will be consumed, assuming no waste?

Ans. 24-26.

4. Calculate the E.M.F. of a Daniell cell from considerations of the heat value of the combinations which take place and the quantity of the elements consumed, taking the heat value for zinc in sulphuric acid as 1670 and that for copper as 909.5.

Ans. 1'11 volts.

5. Describe the construction and working of a modern secondary battery.

6. Most liquids which conduct electricity are decomposed (except the melted metals) in the act of conducting. How do you account for the fact observed by Faraday that the amount of matter transferred through the liquid and deposited on the electrodes is proportional to the amount of electricity transferred through the liquid?

7. Describe the process for multiplying by electricity copies of engravings on wood-blocks.

8. How would you make arrangements for silvering spoons of nickel-bronze by electro-deposition?

QUESTIONS ON CHAPTER XII

1. Sketch an arrangement by which a single line of wire can be used by an operator at either end to signal to the other; the condition of working being that whenever you are not sending a message yourself your instrument shall be in circuit with the line wire, and out of circuit with the battery at your own end.

2. What advantages has the Morse instrument over the needle instruments introduced into telegraphy by Cooke and Wheatstone?

3. Explain the use and construction of a relay.

4. Show, from the law of traction (Art. 384), that the change of attracting force resulting from a change in the number of magnetic lines that enter an armature will be greater if the system is polarized (i.e. magnetized to begin with) than if it is non-polarized.

Ans. Since f N2, it follows that f+df will be proportional to (N+dN)2. Expanding, and subtracting the former, and neglecting the small term (dN)2, we find dfx 2NdN; which shows that, for a given dN, df a N.

5. It is desirable in certain cases (duplex and quadruplex signalling) to arrange telegraphic instruments so that they will respond only to currents which come in one direction through the line. How can this be done?

6. It is wished to make a sort of duplex telegraph by using one set of instruments that work with continuous currents, the other set with rapidly alternating currents, at the same time on the same line. To carry out this idea there must be found (a) an apparatus which will let continuous currents flow through it, but will choke off alternate currents; (b) an apparatus which will transmit alternate currents, but cut off continuous currents. What apparatus will do these things?

7. A battery is set up at one station. A galvanometer needle at a station eighty miles away is deflected through a certain number of degrees when the wire of its coil makes twelve turns round the needle; wire of the same quality being used for both line and galvanometer. At 200 miles the same deflexion is obtained when twenty-four turns are used in the galvanometer-coil. Show by calculation (a) that the internal

resistance of the battery is equal to that of 40 miles of the line-wire; (b) that to produce an equal deflexion at a station 360 miles distant the number of turns of wire in the galvanometer-coil must be 40.

8. Suppose an Atlantic cable to snap off short during the process of laying. How can the distance of the broken end from the shore end be ascertained?

9. Suppose the copper core of a submarine cable to part at some point in the middle without any damage being done to the outer sheath of guttapercha. How could the position of the fault be ascertained by tests made at the shore end?

10. Explain the construction and action of an electric bell.

11. Describe and explain how electric currents are applied in the instruments by which very short intervals of time are measured.

QUESTIONS ON CHAPTER XIII

1. Explain the use of Graham Bell's telephone (1) to transmit vibrations; (2) to reproduce vibrations.

2. Describe a form of telephone in which the vibrations of sound are transmitted by means of the changes they produce in the resistance of a circuit in which there is a constant electromotive-force.

3. Two coils, A and B, of fine insulated wire, made exactly alike, and of the same number of windings in each, are placed upon a common axis, but at a distance of 10 inches apart. They are placed in circuit with one another and with the secondary wire of a small induction-coil of Ruhmkorff's pattern, the connexions being so arranged that the currents run round the two coils in opposite directions. A third coil of fine wire, C, has its two ends connected with a Bell's telephone, to which the experimenter listens while he places this third coil between the other two. He finds that when C is exactly midway between A and B no sound is audible in the telephone, though sounds are heard if C is nearer to either A or B. Explain the cause of this. He also finds that if a bit of iron wire is placed in A silence is not obtained in the telephone until C is moved to a position nearer to B than the middle. Why is this? Lastly, he finds that if a disk of brass, copper, or lead is interposed between A and C, the position of silence for C is now nearer to A than the middle. How is this explained?

QUESTIONS ON CHAPTER XIV

1. What apparatus would you use to produce electric oscillations? Show how you would operate it, and explain why the oscillations take place.

2. Explain how electric oscillations in a condenser circuit produce electric waves in the surrounding medium.

3. The capacity of an air-condenser is 0.001 of a microfarad. It is charged and then discharged through a circuit having a self-induction of 0.004 of a henry and a resistance of 4 ohms. Find the frequency of the vibration. Ans. n = 159,100.

4. Under what circumstances do oscillations not take place when a condenser is discharged?

5. If the frequency of oscillation of a Hertz oscillator is 3,000,000 per second, find the length of the waves it will produce. Ans. 10,000 centimetres.

6. Explain the action of a resonator.

7. Give the reasons which exist for thinking that light is an electromagnetic phenomenon.

8. How is the action of magnetic forces upon the direction of the vibrations of light shown? and what is the difference between magnetic and diamagnetic media in respect of their magneto-optic properties?

9. It was announced by Willoughby Smith that the resistance of selenium is less when exposed to light than in the dark. Describe the apparatus you would employ to investigate this phenomenon. How would you proceed to experiment if you wished to ascertain whether the amount of electric effect was proportional to the amount of illumination?