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Elementary physics laboratory work is given with several purposes in view. Most of the experiments are designed to illustrate to the student the working out of physical principles in a concrete form. Some are planned merely to give practice in the handling of instruments or to show how a certain process is carried out. While the actual results obtained do not have the importance they possess in real research work and in themselves are of small value, still this should never be made the excuse for careless taking of data or the preparation of slipshod reports.
Obtain a notebook of the type required by the instructor and bring it and the laboratory manual to every meeting of the laboratory section.
Before attempting to work an experiment, read the directions carefully, look up the principles involved and have a clear idea of what to do, how to do it, and why it is done. Assignments are made in advance to permit such preparation to be made before the laboratory period.
Begin every report at the top of a page. Form the habit of entering all data at once in the notebook, never on scraps of paper. The original readings should be set down, not merely results calculated from them. Make several trials, whenever possible, record all results, even if the same value is obtained on different trials, and always state the units in which the measurements are taken. Enter also the date, list of apparatus, and any identifying numbers found on the pieces used.
Requirements as to final reports vary in different laboratories, but the following general features are almost universally demanded.
1. The original data must be included as taken.
2. Number and title of experiment, date, etc.
3. List of apparatus, with identifying numbers or marks.
4. A discussion of the experiment. Tell concisely, in your own words, how the apparatus was set up and used. This explanation should be brief, but yet sufficiently explicit to show that you understand the procedure and to permit a person, unfamiliar with the manual used, to comprehend what was done. Use diagrams, wherever possible, to save long descriptions. Do not copy the manual.
5. The data and results computed from them. Unless your original record is in ink and in very good shape, a copy should be made in neat, tabular form, all averages determined and labeled, methods of computation indicated and final results recorded.
6. Conclusion. This may be a comparison of your results with values known to be correct, and an effort to account for any differences which exist. It may be an inference drawn from your work as to the correctness of a physical principle. Very frequently the conclusion shows clearly whether the student has really understood the experiment or has merely gone through the work in mechanical fashion.
7. The report should be ready at the time appointed.
8. In certain cases, the data may be advantageously represented in graphical form. In plotting a curve, the following directions should be observed.
(a) Lay off scales along the horizontal axis (axis of abscissae) and the vertical axis (axis of ordinates), indicating the numerical value of every fifth or tenth division. The two scales need not be the same. Plot only the range of numbers represented in your data, that is, if the numbers run from 75 to 150, there is no need to provide for numbers much below 75 nor above 150. Write along each axis the name of the quantity plotted and the units in which it is measured. A graph should be plotted on such a scale as to occupy most of a page.
(b) Mark by small crosses the locations of all points plotted.
(c) Draw a smooth curve to include as many points as possible. As erroneous results should not lie on the curve, do not try to make the graph pass through points obviously off the line.
A straight line indicates the two quantities plotted are directly proportional to each other; a curve indicates a more complex relation.
EXPERIMENT NO. 101
VERNIER AND MICROMETER CALIPERS: DENSITY. References: Stewart, Physics, Sect. 31, 32; Kimball, College Physics, Sect. 160, 228; Duff, College Physics, Sect. 119; Spinney, Text-Book of Physics, Sect. 101.
(a) The ordinary vernier caliper is an instrument with a sliding jaw for measuring small lengths. A common type has two scales, inches on one edge and centimeters on the other edge of the limb. The small scale on the movable jaw is called the vernier. Close the jaws of the caliper. It will be seen that the ten divisions on the metric vernier have the same length as nine millimeters on the limb. The least count of the instrument is the difference between the lengths of the vernier and limb divisions, in this case, 0.01 centimeter. The student should work out for himself the least count of the other vernier. A large model on the wall is useful in making these relations clear.
Place the metal cylinder in the instrument with the movable jaw pressed firmly against it. Read the number of centimeters and whole millimeters on the limb to the left of the zero-line on the vernier. If this zero-line does not coincide exactly with a division line on the limb, look along the vernier scale until a line is found which coincides with some mark on the limb. The number of this vernier line gives the number of tenths of a millimeter to be added to the limb reading.
Measure the length and diameter of the cylinder, each three times, resetting the jaw between trials. From the average values, compute the volume of the cylinder in cubic centimeters. Weigh the cylinder on the balance, first in one pan and then in the other and take the mean as its mass in grams. Divide the mass by the volume to find the density.
Measure the cylinder in inches and, by comparing the measurements in the two systems, find the number of centimeters in an inch.
(b) The micrometer caliper has a jaw moved by a screw, one turn of which commonly shifts the jaw one-half of a millimeter. A scale parallel to the axis of the screw shows the number of whole millimeters moved. The cylindrical head is divided into fifty parts, so the readings may be taken accurately to hundredths of a millimeter and by estimation to thousandths. If the screw has a pitch of one-fortieth of an inch and the head is divided into twenty-five parts, readings may be taken to thousandths of an inch.
Close the jaws into contact. Use the frictional head in all cases and cease turning when it begins to slip. Take the "zeroreading," which may or may not be zero. Repeat several times. This average zero-reading must be taken into account in finding the true distance between the jaws in any measurement.
Set the steel ball between the jaws and take the reading. Note the number of millimeter divisions uncovered and the head reading coinciding with the longitudinal line of the limb scale. Note carefully whether the screw is in the first or second turn in a given millimeter. If the head reads 35, it indicates 0.35 mm or 0.85 mm, depending on whether the screw is in the first or second turn. Allow for the zero-reading and find the diameter of the ball several times. Weigh the ball, find its volume and then its density.