CHAP. III CHANGE OF STATE 85 Heat so that the bulb of the thermometer is in the water. the flask by your Bunsen burner and notice the thermometer. You will observe that the temperature of the water rises steadily for some time and then stops rising. What is the temperature at which the thermometer stops rising? you notice about the water? What do Raise the thermometer till the bulb is just above the surface of the water, and note the temperature. Place about 50 grammes of common salt in the water, and, after it has been dissolved, take the temperature of the boiling water and then of the steam. In doing this the thermometer bulb should be well cleaned after being in the Fig. 46. If any salt water and before being placed in the steam. salt be allowed to remain on the bulb it will affect the result. What is the effect of the salt on the temperature of the water while it is boiling? What is the effect of the salt on the temperature of the steam which comes from the boiling water? EXPERIMENT 22 Effect of pressure on boiling-point. Using the same apparatus as in last experiment, fix to the end of the escape-tube a short piece of indiarubber tubing capable of being closed by a spring clip. Place the thermometer so that its bulb is just above the surface of the water, and, while the water is boiling freely, note its reading when steady, then close the escape-tube by the clip and notice the thermometer. You will find that it rises. As soon as the thermometer has risen 2° above its reading CHAP. V ELASTICITY 157 one next to it above it. Thus the strain is one in which each little disc slides over the one next to it. EXPERIMENT 31 To find the relation between the elongation of a stretched wire and the force producing it when the wire is stretched within its limit of elasticity. (Hooke's Law.) Take two pieces of wire (brass or copper or steel), about 1 mm. diameter or less, and fix them to a support, so that when weights are attached to their free ends the wires hang side by side a few centimetres apart. The wires should be as long as can be conveniently suspended in this way from some firm support, and it is well to fix them in steel clamps like small vices screwed into a beam in the ceiling. To one wire hang a weight heavy enough to keep it tightly stretched, and to the other attach a scale pan sufficiently large and strong to carry weights up to 20 or 30 kgm. The former wire should be shorter than the latter, so that the two wires may be able to hang close together without the shorter wire being touched by the scale pan. Extra Crown 8vo. 862 pages. 7s. 6d. AN ELEMENTARY COURSE OF PHYSICS EDITED BY REV. J. C. P. ALDOUS, M.A. CHIEF INSTRUCTOR TO NAVAL CADETS H.M.S. BRITANNIA, 1875-98, Or, in Three Parts Part I.-MECHANICS, PROPERTIES OF MATTER, HYDROSTATICS, Part II.-WAVE MOTION, SOUND, LIGHT. 4s. 6d. By W. D. Eggar, M.A., 220 pp. 2s. 6d. Part III.-MAGNETISM, ELECTRICITY. By F. R. Barrell, M.A., B.Sc., Prof. of Math., Univ. Coll., Bristol. 230 pp. 2s. 6d. London MACMILLAN AND CO., LIMITED NEW YORK: THE MACMILLAN COMPANY X NATURE AND SOURCES OF HEAT 367 There may motion which we call Heat; it is not momentum. have been no change in the momentum of the apparatus; yet there has been a gradual development of Heat. What then has been done? Work has been done on the handle of the whirling table which has been pressed with a definite force through a definite distance. The 'Principle of Work' applies only to 'perfect' machines. In a real machine some work is done against friction, so that in practice the work done on a machine is not equal to the work done by the machine. The work done against friction produces kinetic energy in the form of heat. Heat and Kinetic Energy.-A metal plunger fits well in a strong metal cylinder, at the bottom of which is placed a piece of tinder. Now if the plunger be put in the cylinder so as to enclose some air, and then be smartly pushed in, or better still, hammered in smartly with a heavy hammer, the air inside 722 ELECTRICITY-VOLTAIC CHAP. wire in the same direction as the current; then the N. pole of the needle is urged round the wire the same way as the point of the corkscrew moves. A Fig. 56.-Screw movement of north pole and current. Simple Galvanometer.-Bend a piece of stout copper wire into a rectangular shape as ABCD, and connect with the wires Fig. 57.-Action of current flowing round a rectangle. from a simple voltaic cell; suspend a magnet needle in the middle, then the dolls as sketched show that all four sides, AB, |