of the interior of the cylinder, not occupied by the coil, was filled with sand. The other terminal of the coil, after being well insulated, was pulled through a hole in the detachable end of the cylinder and this end fastened in place. Mounted on the shaft, but well insulated from it and from the cylinder, are two brass rings, one at each end of the cylinder. To each of these rings is attached one terminal of the german silver wire coil. Brushes for supplying current to the coil within the cylinder bear upon these rings. On the exterior of the cylinder a layer of muslin was wrapped, the muslin being well soaked with shellac. A coil of copper wire (No. 18 A. w. G.) was then wound on the cylinder. Attached to each end of the cylinder, but insulated from it, is a brass ring. The ends of the copper wire coil (which is wound on the exterior of the cylinder) are attached to these rings. Brushes bear on these rings and supply current to the exterior coil. The object of this latter construction is to measure the temperature while the cylinder is revolving. The temperature of the cylinder at any moment can be measured by finding the resistance of the copper wire coil. The temperature coefficient of the wire being obtained by previous experiment. A current is sent through the interior coil, and heat is thereby generated; the energy thus transformed into heat being measured by the product of the resistance of the coil and the square of the current flowing through it. Or as the resistance is a variable quantity, the energy is better measured by the product of the current flowing through the coil and the difference of potential at its terminals. By placing an ammeter in the circuit and connecting a voltmeter to the terminals of the coil, we can measure the exact rate at which heat is developed, and thus the total quantity of heat developed during any period of time. The heat produced, raises the temperature of the wire, and thus that of the cylinder. The method pursued was to make several series of runs; the amount of the cylindrical surface of the cylinder covered by the field being the same for each series, while the speed was varied from zero to about 2000 revolutions per minute, the speed being kept constant during each run. Four speeds were used, viz.3000 ft. per minute, 2000 ft. per minute, 1000 ft. per minute and a fourth series with the cylinder at rest. Each of these series will show the effect of different peripheral velocities. To show the effect of the field in aiding or in preventing the radiation of heat, four sets of fields were used;-one covering 100% of the cylindrical surface of the armature, another covering 75%, a third covering 50% and a fourth run with no fields at all. A pair of 25% fields were also made but were not used because of the comparatively slight difference between the radiation with no fields and that with 50% fields. The object of the above method is to obtain curves showing the effect of speed, and another set showing the effect of the field in preventing or in aiding the radiation or conduction of heat from the armature surface. To find the relation of the amount of heat radiated to the temperature of the cylinder, the rate at which heat is developed in the cylinder was varied from 100 to 400 watts, the rate being kept as near constant as practicable during each run. An examination of the above will show that a series of runs was made with each pair of polepieces and that for each run in the series, a second series was made with different peripheral velocities, and again for each peripheral velocity a series of runs was made, each with a different amount of heat developed per second within the cylinder. The method used in each run was as follows:-Being unable to obtain the requisite instruments we were compelled to abandon the proposed method of finding the temperature of the cylinder, and substitute a thermometer. While this method is not quite as accurate as that intended when the apparatus was built, nevertheless it gives very good results, as can be seen from the figures obtained, which agree very well when considered together. A current was sent through the coil within the cylinder and heat produced, thus raising the temperature of the cylinder. The rate at which heat was developed was kept constant during each run. To do this it was found necessary to place a resistance that could be easily varied in series with the coil, for, owing to the great range of temperature the resistance of the coil varied greatly. Runs were to be made long enough to allow the cylinder to reach constant temperature; for when the temperature becomes constant the amount of heat radiated must be exactly equal to the amount generated. Hence we have a very accurate method for determining the amount of heat radiated from the surface of the cylinder. However, one great difficulty was encountered; being unable to make use of the coil on the exterior of the cylinder, how were we to ascertain when the cylinder had reached constant temperature? A number of runs were made, and although continued for over three hours the results obtained did not agree as well as was expected. The reason for this disagreement it was thought was, that the runs were not long enough, that is the temperature of the cylinder had not become constant when the cylinder was stopped and the temperature taken. To ensure that the temperature had become constant, it was found best to adopt the following method:-The cylinder was allowed to remain at rest or revolved slowly, and a current sent through it until the temperature had become from 5% to 10% higher than the calculated temperature. The run was then started and continued for a length of time varying from one and a half to three hours, according to the conditions of the run. By the calculated temperature is meant that calculated from runs which had been continued long enough to ensure constant temperature, in some cases over four hours. By this method greater accuracy was obtained and we were more certain of our results. To obtain the temperature of the cylinder a thermometer was placed on the cylinder and the bulb covered with a piece of cotton waste, the cylinder being first brought to rest. In order to obtain the temperature as quickly as possible after the cylinder had been brought to rest, it was found advantageous to heat the thermometer to a temperature higher than that at which the cylinder was calculated to be, and allow it to fall to the correct tempera ture. The temperature of the atmosphere was also taken. The difference between the two is the rise in temperature due to the heat generated within the cylinder. Following is given a table of the data obtained by pursuing this method: 'Two sets of curves were plotted from the above data. In each set the actual rise in temperature of the cylinder was plotted along the vertical. For one set Fig. 2. the total amount of heat radiated per second was plotted along the horizontal, while in the second set Fig. 3, the speeds were plotted as abscissæ. Smooth curves were drawn through the points or near to them; the two sets of curves forming a check upon each other aided greatly in drawing the curves correctly. In the next table are given the corrected results as obtained from the curves, and also the amount of heat radiated per square inch per degree rise in temperature. The latter quantity was obtained by dividing the total amount of heat radiated (in watts) by the product of the rise in temperature and the area of the 1. One group in each set is shown. |