each other. See that readings of the surface of the water can be taken on the graduated scale of the float, both when it is raised by the tension of the thread, and when the thread is quite slack. The difference between the two readings should be 5 to 10 cms. This can be secured by using a little petroleum as lubricant between the cones if the difference is too great, and a little vaseline if it is too small. Take the reading of the float when the tension in the thread is zero. Read the two dials attached to the rotating apparatus, and measure the length of the wooden rod from the centre of the cones to the point of attachment of the thread. Measure the diameter and length of the bulb of the thermometer and calculate its water equivalent. Hang the thermometer on the movable arm attached to the stand and lower the bulb into the inner cone till it is below the level of the mercury. Take observations of temperature every half minute for 3 minutes, then wait 3 minutes and take observations from 3 more minutes. At the end of this interval commence to rotate the hand wheel steadily, continuing to observe the temperature every half minute. At the middle of each half minute take a reading of the surface of the water on the float. Continue the rotation till the temperature has risen about 5°C. Then stop the rotation, read the temperature till the change has been regular for 3 minutes, wait 3 minutes, and read again for 3 minutes. Read the dials and the float. Remove the float from the water and determine its crosssection at 3 or 4 points, between where the readings have been taken. Take the mean of these determinations. Take also the mean of the readings of the float in the raised position, and subtract from it the mean reading for no tension. The product of this difference into the mean cross-section of the float is the mean tension in gravitation measure. Multiply this by 2 times the number of revolutions and by the length of the wooden arm and by g, and the product is the work done during the rotation in ergs. Apply the correction for cooling to the temperature readings as in Section XXV. and determine the corrected rise of temperature. The product of this by the water equivalent of cones and contents is the heat generated in gram-degrees, ¿.e. in terms of the unit of heat to which the specific heats used have been referred. The quotient of the number of ergs of work done, by the number of gram-degrees of heat generated, is the number of ergs work required to generate heat sufficient to raise 1 gram of water 1° C. Arrange your observations and results as follows: Total water equivalent of cones and contents Readings during rotation: 19.70, 1960, 19.55... &c., mean Mean rise of float ... ... ... ... 19.52 = ... ... ... ... ... Diameter of float stem = 2.02, 2·01, 2·00, 1.98, 1.99 Mean cross section = ... 25.7 cms. 3.14 sq. cm. .. Work done = 25·7 × 13 × 3·14 × 3532 × 981 = 363 × 107 ergs. Temperature at end of first period Mean temperature at end of third period) after correcting for cooling Rise of temperature Heat generated Mechanical equivalent Repeat the experiment, and if the two results are nearly alike take the mean as the final value. S. P. 10 BOOK IV. SOUND. SECTION XXXI. wren FREQUENCY OF A TUNING FORK BY THE SYREN. Apparatus required: Tuning-fork, bow, singing flame, syren and blowing apparatus. To enable the comparison of a fork and a syren to be made more conveniently than it can be done directly, it is usual to tune a singing flame to the fork by adjusting the length and position of the resonating tube over the flame, and then to tune the syren to the flame. Calculate the length of an open pipe which will act as a resonator to the fork, the vibration frequency of which is supposed to be known roughly. Take a glass tube of rather less length, and roll a piece of paper round one end so that the effective length of the tube may be altered by sliding the paper tube along. Place the tube above a small gas flame produced by gas issuing from a minute hole at the end of a conical glass tube. Bring down the pipe on to the flame, and adjust till the flame "sings"; then vary the position of the paper tube till the note emitted by the pipe produces no beats with the note of the fork (Fig. 55). Place the syren on the blowing apparatus, start the apparatus and increase the rate of blowing till the note emitted by the syren produces no beats with the pipe. Fig. 55. Maintain the rate of blowing, and at a given instant take a reading of the positions of the fingers on the dials indicating revolutions of the spindle of the syren. At the end of a minute again take readings. Subtract the readings to get the number of revolutions in the interval, count the number of holes in the disc of the syren, find the product and divide by 60, the result is the frequency of the note of the syren, and hence of the pipe, and fork. Tabulate your results as follows: 10 November, 1898. Fork "A" marked 256. Open pipe to act as resonator = 34000/512 = 66 cms. Reading on syren dials at beginning of minute 300, 220, 410 SECTION XXXII. VELOCITY OF SOUND IN AIR AND OTHER BODIES BY KUNDT'S METHOD. Apparatus required: Kundt's apparatus, rods and rubber. Kundt's apparatus consists of a glass tube of about 200 cms. length and 5 cms. diameter into one end of which a rod of wood, metal or glass projects (Fig. 56). Fig. 56. The rod is securely clamped to the table at its middle point, and can be set into vibration parallel to its length by stroking it, if wood with a piece of cloth on to which a little resin has been rubbed, if metal with a leather rubber similarly treated, or if glass with a damp cloth. The end which projects into the tube is provided with a cardboard disc which has a diameter a little less than that of the inside of the tube. The tube contains a little lycopodium powder or finely divided cork and the further end is stopped by a movable plug. It is important that both tube and powder should be quite dry. When the rod is set into logitudinal vibrations, the disc on the end in the tube sets the air in the tube in vibration and the powder is carried along with the air. If the tube is long enough, there are certain parts of it where the air is not in |