« PreviousContinue »
EXPERIMENT NO. 408
RESISTANCE BY VOLTMETER-AMMETER METHOD.
References: Stewart, Physics, Sect. 400, 401, 434; Kimball, College Physics, Sect. 603-605; Duff, College Physics, Sect. 317, 323; Spinney, Text-Book of Physics, Sect. 316, 320, 330.
According to Ohm's Law, the resistance of an electrical conductor is defined as the ratio between the potential difference applied to the conductor and the current flowing through the conductor. Direct current should be used, as the rule does not apply to alternating-current circuits except under special conditions. If an ammeter is connected in series and a voltmeter in parallel with the conductor, the current and potential difference can be determined and the resistance computed with a fair degree of accuracy.
An interesting experiment is to use this method to measure the resistances of the filaments of carbon and tungsten incandescent lamps under different conditions of filament temperature. The apparatus is to be set up as in the diagram. DF is a slide rheostat connected through a switch to D C terminals. A branch circuit is connected from D through the lamp and ammeter, A, to the sliding junction, C, which can be placed anywhere along DF. The voltmeter, V, is connected directly to the lamp terminals.
Caution. The student should not finally close the dynamo circuit till the connections have been approved by the instructor; otherwise, if the ammeter should be wrongly connected, it might be ruined.
Now beginning with C near the end, D, record simultaneous readings of the voltmeter and ammeter, placing C at such intervals along DF that the voltmeter readings differ by about 5 volts. Since RE/I, we can compute the resistance of the lamp for any voltage. The readings of the ammeter must be taken very carefully, fractions of a division being estimated, especially when the current values are small.
Plot for each lamp a curve, using potential differences as abscissæ and corresponding resistances as ordinates. If an old carbon lamp is used, its resistance will vary in a decidedly different manner from that of the tungsten. Look up the effects of increased temperature on the resistances of metals and nonmetals respectively. If a "metallized" filament is in the carbon lamp under test, there will not be the pronounced difference in resistance variation mentioned above. State the effect of temperature changes on resistance.
EXPERIMENT NO. 409
References: Stewart, Physics, Sect. 451-456; Kimball, College Physics, Sect. 607-617; Duff, College Physics, Sect. 342351; Spinney, Text-Book of Physics, Sect. 336-340.
Faraday's laws of electrolysis state that the mass of a substance liberated by a current of I amperes flowing through a solution for t seconds is,
In this experiment the law is tested for hydrogen, oxygen and copper.
Direct current from some source is sent through a resistance, a low-range ammeter, a water coulommeter and a switch connected in series. The water must contain a little sulfuric acid. Usually the apparatus is given out filled with the proper solution. Allow the current to run till enough gas is in each burette to permit a reading to be taken. Record this as the zeroreading, and later subtract it from the final reading of that burette. After this is done, çlose the switch, noting the exact
time, and let the current run till the hydrogen tube is nearly full, recording ammeter readings every two minutes. Note time of opening the switch, compute the number of seconds elapsed and the average current, find the room temperature, the barometer reading and the water head in the coulommeter acting on each gas. After the tubes have cooled for a few minutes, read the volumes of the two gases.
Clean the copper cathode plate thoroughly with fine sandpaper and weigh it carefully to thousandths of a gram on the best balance available. Connect the copper coulommeter (filled with copper sulfate solution) in the place of the water coulommeter, insert the weighed cathode plate and let the current run for 30 to 45 minutes, noting elapsed time carefully and taking frequent readings of the ammeter as before. At the end of the run, rinse the plate in running water without rubbing it, flow alcohol over it, dry quickly in a current of air or over a radiator and weigh it again as accurately as possible. Find the gain in
Calculate from the time, mean ammeter readings, and chemical equivalents of the substances, the masses of each which should have been set free, using formula (1). Compare with your observed results. To make this comparison for the gases, it will be necessary to determine the masses of the volumes of hydrogen and oxygen collected in the burettes. One thousand cubic centimeters of hydrogen at standard temperature and pressure weigh .09004 grams, one thousand cubic centimeters of oxygen 1.4292 grams. Reduce the measured volumes to standard conditions and find their masses. In correcting for pressure, add to the barometric reading the height of mercury equivalent to the water head in the coulommeter and subtract the proper amount for saturated water vapor pressure at the existing temperature. (See tables for this).
EXPERIMENT NO. 410
MAGNETIC FIELDS OF CURRENTS.
References: Stewart, Physics, Sect. 405, 406, 462-466, 488; Kimball, College Physics, Sect. 671-677, 684, 748-749; Duff, College Physics, Sect. 288, 302-304, 311, 335, 362; Spinney, Text-Book of Physics, Sect. 240, 303-305, 309-311.
This experiment is designed to show, in a qualitative way, some of the facts regarding the magnetic fields about currentcarrying wires. All of the circuits described can be mounted on one frame with suitable binding posts for each circuit, or, if desired, they may be mounted separately.
Connect to the terminals of the 110-volt direct-current circuit a reversing switch, a lamp rheostat and a large, vertical, rectangular coil, all in series. One side of the coil is passed through a hole in a glass plate or a sheet of heavy cardboard. Sprinkle some iron filings over the plate, while the switch is open, and tap the plate. Note whether the filings lie at random or in some definite arrangement. Now turn on a fairly strong current and again tap the plate lightly. Place a small compass on the plate and find the direction of the magnetic lines in the field. Compare results with the indications given by the righthand rule. Try the effect of reversing the current. Describe the arrangement of the lines and the direction of the field for both directions of the current. Make sketches of the two fields and indicate on each by arrows the directions of the current and the magnetic lines.
Replace the large coil with a smaller one, both sides of which pass through a horizontal plate. As before, determine the appearance of the magnetic field by sprinkling iron filings over the plate, and use the compass to find the direction of the field. Reverse the current and note the effect. Test the right-hand rule each time. Make sketches to illustrate the two cases.
Remove the coil from the circuit and replace it with a single strand of fine wire or a single strip of thin tin-foil. This wire should hang loosely in a vertical position. Hold a strong bar magnet horizontally and bring the north-seeking pole close to the wire, then turn on the current and notice the action of the
wire. The magnetic lines from the north-seeking pole of the magnet produce about the wire a magnetic field which is horizontal and directed away from the pole. The magnetic field. due to the current in the wire is in the same direction as that of the magnet on one side of the wire and in opposition to it on the other side. Is the wire deflected toward the side where the two fields act together or toward the side where they oppose each other? Note carefully that the wire is neither repelled nor attracted but pushed to one side or the other. Repeat the test with the current flowing in the opposite direction through the wire.
Bring the north-seeking pole of the magnet near the loop of the filament of a carbon incandescent lamp on a direct-current circuit and note what happens to the filament. Then change to an alternating-current circuit and note the action of the filament when the magnet pole is brought near. What does this show about the direction of an alternating current?
EXPERIMENT NO. 411
THE GALVANOMETER AS AMMETER AND VOLTMETER. References: Stewart, Physics, Sect. 467-469; Kimball, College Physics, Sect. 692, 694, 697; Duff, College Physics, Sect. 314; Spinney, Text-Book of Physics, Sect. 374, 377, 380. Most of the ammeters and voltmeters used for direct-current measurements are moving-coil galvanometers fitted with suitable shunts or series resistors and so calibrated as to read directly in amperes or volts or fractions of these units. It is the purpose of this experiment to show how a moving-coil galvanometer of the type read by means of a telescope and scale may be calibrated as an ammeter and as a voltmeter.
(a) The ammeter-In the ordinary galvanometer a very small current produces a large deflection, so that in order to use this instrument for measuring large currents a shunt must be placed across the terminals. The instrument then becomes an ammeter.
Connect up a circuit as shown in the diagram, using a gravity cell or storage battery. S is a shunt of low resistance