EXPERIMENT NO. 405

POWER AND EFFICIENCY OF AN ELECTRIC MOTOR.

ences: Stewart, Physics, Sect. 80, 81, 90, 418, 419; Kimball, College Physics, Sect. 65, 66, 96, 700; Duff; College Physics, Sect. 15, 16, 328; Spinney, Text-Book of Physics, Sect. 57-59, 74, 85-87, 327, 373, 380.

Power is the time rate of doing work; the common units he horse power or 33,000 foot pounds per minute and the which is 1 joule per second. The watt is also equal to the r furnished by a current of 1 ampere in a conductor over the potential drop is one volt. Efficiency is defined as atio between the output of useful work and the total input ergy. It may also be computed from input and output

r.

With a direct-current motor, we may find the input power king the product of current flowing and the applied voltage, with an alternating-current machine, it will be necessary to watt-meter.

The output power is usually found by means of a brake

The strap-brake is the simplest type. A cord or strap es around the pulley on the shaft of the motor and its ends astened to two spring balances suspended from a bar above notor. Another arrangement is to use a spring balance at end of the band and to run the other end over a pulley to le-pan on which weights may be placed. This arrangement mewhat easier to read. The frictional force exerted at the of the pulley is given by the difference between the balance ngs or between the balance reading and the weight on the end of the cord (including the scale-pan). The distance force moves per second may be found as follows: Measure

the diameter of the pulley and compute the circumference or measure the circumference directly, find the number of revolutions per second the motor is making with this load and multiply by the circumference. This distance per second in centimeters multiplied by the frictional force in dynes will give the output in ergs per second, and the watts may be found by dividing by 10,000,000.

Connect to 110-volt D C terminals in series a switch, the motor and an ammeter of suitable range. In parallel with the motor terminals connect a voltmeter of 120 or 150 volt range. There should be fuses in the line. If the motor is of large size, a starting rheostat will also be needed. Have the connections approved by the instructor before turning on the current. Measure the diameter or circumference of the pulley and record. Learn how to use the revolution counter. To avoid violent jerks on the brake band, it is best always to start the motor with a small load and apply greater force after it has come up to speed. The weight and pulley arrangement makes this very easy as one can lift the weight at the start and lower it after the motor is running. Make the first test with a small load. When the motor is pulling the load steadily, read and record indications of the voltmeter, ammeter, spring balance and weight (or both spring balances) and find the number of revolutions in a minute. Owing to the large amounts of heat generated in the brakes at high load especially, it may be well to apply the speed counter for only 15 or 30 seconds rather than for a full minute. As soon as the readings have been taken, OPEN THE SWITCH. It may be necessary to detach the pulley after each test and cool it in water. Then repeat with a larger force applied, but remember to start with load released. Make a number of tests with gradually increasing load, till eight or ten trials have been made. Ask for instructions as to the safe limit.

run.

Compute input power, output power, and efficiency for each Plot a curve using efficiencies as ordinates and output watts as abscissae. What does this curve show about the gain in efficiency to be obtained by increasing the load? If any point on the plot lies far off the curve indicated by the rest of the data, it will be well to repeat that run as nearly as possible and find if any error has been made.

EXPERIMENT NO. 406

JOULE'S LAW.

References: Stewart, Physics, Sect. 427-429; Kimball, College Physics, Sect. 654, 655; Duff, College Physics, Sect. 328, 329; Spinney, Text-Book of Physics, Sect. 325-327.

Joule's Law states that the heat produced in any conductor is proportional to the resistance of that conductor, to the square of the current, and to the time the current flows. If the heat energy is expressed in calories, the resistance in ohms, the current in amperes and the time in seconds, the following equation is true;

12 R t

H

J

J is the mechanical equivalent of heat and should be 4.187 joules per calorie in this case.

The purpose of this experiment is to determine the mechanical equivalent of heat from measurements of the heat produced and electrical energy used by an incandescent lamp.

A lamp is mounted in a socket fastened to the under side of the wooden cover of a calorimeter. The calorimeter should be encased in a jacket to prevent heat losses as far as possible.

Connect the terminals of the 110-volt circuit through a switch and an ammeter to the terminals of the incandescent lamp which is to be placed in the calorimeter. Across the terminals of the lamp connect a voltmeter, as shown in the diagram. Have the instructor approve your connections before you turn on the current. Weigh the calorimeter and stirrer and find the water equivalent as previously directed. Find also the water equiva

lent of the immersed part of the thermometer. Weigh out in the calorimeter enough water to cover most of the bulb when the cover of the calorimeter is in place, but do not permit any water to get into the socket or on the cover. The water should be several degrees below room temperature at the start and the heating should be continued till the temperature has been raised 15° or more. The water equivalent of the glass part of an ordinary 50-watt carbon incandescent lamp is about 4.75 grams.

Set the lamp in place and read the temperature of the water after thorough stirring. Turn on the current, noting the exact time the switch is closed, and continue the heating till the desired change in temperature has taken place. Read the ammeter and voltmeter every minute. When the temperature is high enough, open the switch, note the exact time, stir the water and take the temperature as soon as possible.

From the rise in temperature, the mass of the water, and the various water equivalents, compute the number of calories of heat produced. Find the average current and voltage used and from them compute the resistance of the lamp. Determine the number of seconds the current flowed through the lamp. As all quantities in the equation are known, with the exception of J, the mechanical equivalent of heat can be computed.

Make at least two trials.

EXPERIMENT NO. 407

THE SLIDE-WIRE BRIDGE.

References: Stewart, Physics, Sect. 436-439; Kimball, College Physics, Sect. 641, 642, 647, 649, 650; Duff, College Physics, Sect. 322, 324, 325; Spinney, Text-Book of Physics, Sect. 321, 323.

The purpose of this experiment is to measure resistances by means of the slide-wire Wheatstone bridge.

In the accompanying diagram AB represents the slide-wire, R, a resistance box of which the resistance may be varied, and X is the resistance to be measured. A battery and a galvanometer, G, are connected as shown. D is the slider which moves along the wire, AB, K a tapping key in the battery circuit.

meter shows a reversed deflection. This means that R is rger than X. Repeat with smaller values of R until two restances are found, differing by one ohm, which give opposite flections of the galvanometer. With the smaller of these sistances in R, move the slide-key to such a position that the lvanometer shows no deflection when first K and then D are osed. Then the resistance, X, is given by the equation,

crease R by one ohm and make another trial, then interchange and R and repeat. In this way measure the resistance of each three coils separately.

Measure the resistance of the three coils joined in series. his should equal the sum of the separate resistances of the ree coils. Add the separate resistances to see how well your sults check.

Measure the resistance of the coils when connected in parallel. he reciprocal of the combined resistance should equal the sum the reciprocals of the separate resistances. Show how closely ur results check.