ENERGY RELATIONS OF DYNAMO AND MOTOR. High School of Commerce, New York City. The dynamo and motor are reversible; a d. c. dynamo may be operated as a motor and a d. c. motor may be operated as a dynamo. This reversibility results not only from their similarity of construction but from the fact that in every operating dynamo the motor principle is likewise operative and in any operating motor the dynamo principle is operative. The power driving a dynamo comes from some external prime mover, but the forces opposing the rotation of the armature are internal. We say that it requires more power to drive a dynamo when the external load is increased but it is not so obvious as when in lifting a weight with a plank as a lever we increase the weight lifted from (say) 50 to 100 lbs. The greater external electrical load of a dynamo results in greater internal forces opposing the rotation of the armature. The current which a dynamo delivers magnetizes both field magnet and armature and the mutual action of the two magnetic fields thus produced results in forces which oppose the external prime mover. This may be made more apparent perhaps if we use the two threefinger rules, the right hand for dynamo action and the left hand for motor action. Hold both arms in front of you, extend both forefingers away from you to represent the direction of the field flux, extend both middle fingers at right angles to the forefingers and to the right to represent the current flowing in the armature conductors, then you see that one thumb points upward and the other downward. The right thumb shows the direction in which the external prime mover is driving the armature conductor and the left thumb shows the direction of the internal motor forces acting upon the same armature conductor. Thus we see that in the very act of getting more electrical power out of a dynamo there are established within the dynamo correspondingly greater forces opposing the driving power, causing an automatic increase in the mechanical intake of power. You cannot get something for nothing; energy is neither created nor destroyed; the electrical output of power of a dynamo cannot exceed its mechanical intake of power. The driving forces of an electric motor are internal, although the power is obtained from an external generator, and the forces opposing these driving forces are external, resulting from the mechanical load. The forces opposing the rotation of a trolley car motor when the car is moving up grade are greater than when the car is moving on a level track and the motor takes in from the external generator a correspondingly greater power. Why should the same external voltage (say 550) send more amperes into the same motor? The answer is not apparent until you realize that the dynamo principle is operating in the motor. = The dynamo principle is: If a wire is moved so as to cut lines of magnetic force, an e. m. f. is generated in that wire. In a motor armature are conductors moving across magnetic flux, hence there must be an e. m. f. generated in the armature conductors. If we use the same two three-finger rules and point the thumbs up and both forefingers away from you, one middle finger points to the left and the other to the right. The left middle finger shows the direction of current actually flowing in the motor conductor and the right middle finger shows the direction of the e. m. f. generated in that conductor by dynamo action. Thus we see that the dynamo action in the motor armature opposes the flow of current in the motor. This opposition to the motor current is not an ohmic resistance; it is a counter voltage. Ohm's law, p. d. ir, is not true for an electric motor; the true formula is p. d. ir+Em, where Em represents this counter voltage. Since this counter voltage is produced by dynamo action, its magnitude is determined by speed of armature, intensity of flux and number of armature conductors. We now may show why the same voltage will send more amperes through the same motor at one time than another. If a 550 volt, 14 ohm motor on a level track is moving at such speed that its counter voltage is 544, the excess of 6 volts will send 24 amperes through the motor. As soon as the car strikes the up grade, the greater external mechanical resistance will immediately lessen the speed of the motor and its counter voltage will immediately lessen to some value less than 544 (say 540). The excess voltage which now becomes 10 will send 40 amperes through the motor creating sufficient internal driving force to move the car up hill. = Hence it is apparent that in the very act of getting greater mechanical work done by a motor, conditions are established which cause a greater intake of electrical power. You cannot get something for nothing; energy is neither created nor destroyed; the mechanical output of power of a motor cannot exceed its electrical intake of power. THE USE OF LOCAL APPLICATIONS IN THE TEACHING OF PHYSICS.1 BY S. E. Boomer, Southern Illinois Normal School, Carbondale, Ill. The subject of physics' is elective in many schools. I suppose we are not unique in having pupils whose chief plank in their school platform runs about as follows: We hold it to be our sacred right and our patriotic duty with which no autocrat, be he high school principal or other authority, will be permitted to interfere, to find and to follow the line of least resistance through school. They have heard from former students in physics of the hard problems, difficult laws, laborious reports and they vote another ticket. Indeed, many earnest students fearing these difficulties and seeing no special need for the subject elect an easier course. We may believe that physics is the most important subject in the curriculum. We may believe in it as strongly as we believe in the four gospels but if pupils in our school elect another route we have little opportunity to convert them to our faith. We could not argue, if we would, that physics offers a line of least resistance. Let us not try to popularize it by making it easy. I have little sympathy with those teachers who would try to sugar coat every lesson and avoid all difficulties in order to shield the dear boys and girls from real work. I want my pupils to have some experience with accurate measurements. I want them to know something of the methods of science and have a high regard for its conclusions. I want them to understand that scientists are among the hardest workers, and that it is necessary for them to dig if they would accomplish anything worth while. But I should like to have them enjoy the digging. It has been a pleasure to me to observe that teachers of physics are not ashamed to describe in our meetings and in Science and Mathematics the simplest piece of apparatus or give to their fellow teachers a most elementary device in teaching. It is because of this spirit among us that I presume to ask your indulgence while I relate some very simple but delightful experiences. Primarily most of us are not investigators except in the sense that we are seekers after the best methods of teaching our great subject. If we can find some way, however simple it may be, to make our teaching more stimulating, more invigorating, more Read before the Physics Section of the C. A. S. and M. T. at Soldan High School, St. Louis, Nov. 25, 1921. attractive, more useful. or more practical without sacrificing any of its value it is well that we should pass it on. No doubt many of you in industrial centers have done far more than I have in the use of local applications. My hope is that these suggestions may be of some service to teachers in communities which have few industries. Many teachers of physics in such communities do all their work in lecture or recitation room and laboratory. They do not realize that there is abundant and inspiring material outside. Our little city of 6,500 inhabitants is not an industrial center, but we find much more material than we can use in one class. Seven miles out from town Mr. Charles Etherton installed a water ram at a spring to furnish water for his farm house and barn. We went over the hills to see this single application. The principle of the ram was discussed while we listened to the clicking of that one at the spring. We measured the amount of water flowing through the ram per minute, 14 pints, and the amount delivered to a trough at the barn per minute, 1 pint. Mr. Etherton gave us the distance from the ram to the house, 1,000 feet, and the elevation, 85 feet. He explained how water was delivered to various places by the operation of valves. We computed the amount of water delivered per day and the amount of work done. There is another spring near by. The class estimated the amount of water flowing from the two, the fall that could be obtained and decided that sufficient energy was available to furnish electricity for the home. As we walked back up the steep, rocky path Mr. Etherton remarked, "For forty years my mother walked up this old rocky path in dry seasons with a kid in one arm and a bucket of water in the other hand." The boys and gir's gained a better appreciation of this one application than I could possibly have given them with our pretty little glass model in the laboratory. Here was physics acting as a labor saver, not a labor maker. Our genial host took us to his fine orchard and told us of an interesting example of air drainage and frost. The east side of the orchard sloped rapidly down into a woodland which continued to a ravine far below. One year all the apples in the lowest row were destroyed by frost. On the next row only those at the very top were left. Farther up the slope only the lower half were destroyed, then only those on the lower branches were claimed by the frost. Above this row all the trees hung heavy with Winesaps and Johnathans. I asked him to let the class work out the explanation. They did so with much more interest than I have been able to secure with the best book problems. Ayer and Lord have in Carbondale the largest tie preserving plant in the world. Several times I have asked the superintendent for the privilege of having my classes visit the plant. It has always been granted gladly and a foreman has given his time to show us through and explain to a fascinated class the various operations. We saw many pumps, large and strongly built. Our glass models have their value but here the class saw one pump compressing air and steam at 180 pounds to a square inch into a cylinder 120 feet long and 8 feet in diameter, another exhausting the steam and air from a similar cylinder, a third forcing the creosote into the pores of seven hundred ties in a third cylinder of like dimensions with a pressure of 150 pounds to the square inch (Pascal's Law). They observed a train of little cars loaded with ties being pulled into another big cylinder by electric motor. The ties were bound down for, by Archimedes' principle, they would float in the liquid. The foreman called our attention to a hydraulic press he had made for forcing the little car wheels on the axle, the hole in the wheel being three thousandths of an inch less in diameter than the axle. The press had thick walls capable of sustaining a force of thirty tons which may be exerted by the machine. Another example of Pascal's Law. The method of heating the creosote furnished a fine application of convection. Two tanks twelve feet long and one foot in diameter were mounted below the 23,000 gallon creosote tank at an angle of about 45 degrees with the vertical. Through each of these passed pipes 158 feet long wound into coils carrying steam under high pressure. The lower ends of the small tanks were attached to the bottom of the big tank and the upper ends to points near the top of the latter. The foreman told us that when the hot steam was flowing through the coils the creosote flowed through these small tanks faster than any pump could force it through. We observed hastily their big engines, dynamos, motors and other applications of physics. Time did not permit a discussion of these. A happy class returned to the laboratory. They were not required to "write it up." They did not understand all they saw but they have a new vision of the wonderful field in which they have begun to study. Our electricity is furnished by the Central Illinois Public |