We come now to a consideration of the effect of the field in the amount of heat radiated or liberated from an armature surface. From the set of curves just discussed, points were taken at 25 degrees rise, at 50 degrees rise, at 75 degrees rise and at 100 degrees rise in temperature and another set of curves plotted from these points with the amount of heat liberated per sq. inch degrees rise in temperature as ordinates, and the per cent. of the cylindrical surface of the armature covered by the poles is plotted along the horizontal. In Fig. 12 are plotted curves for a peripheral velocity of 3000 ft. per minute. Curve I shows the relation between the amount of heat liberated per sq. inch per degree rise in temperature and per cent. of cylinder covered by the poles when the temperature of the cylinder is 100° centigrade above that of the atmosphere. Curves II, III and IV show similar relations when the rise in temperature is 75°, 50° and 25° respectively. All of these curves are similar in character and are very nearly straight lines. In fact for all practical purposes we would not be far wrong in calling them such. Curve II is perhaps of the most value of the four, as a rise of 75° C. above the temperature of the dynamo-room may be assumed as a safe rise in temperature of an armature. With no field, we find that the amount of heat liberated per sq. inch per degree rise in temperature is 0.0275 watts and with the 100 per cent. field the heat radiated is only 0.0222 watts per sq. inch per degree rise in temperature; which gives a variation due to the field of about 20% in the amount of heat liberated by the armature. That is, the amount of heat liberated with the 100% field is 20% less than the amount liberated when no field is used. This great variation should be of value to the engineer in the design of electrical machinery.

In Fig. 13 we have a set of curves similar to the last in construction, but entirely different in shape. The peripheral velocity in this case is 2000 feet per minute. We no longer find curves of single curvature, nor do they (except No. IV.) even approach straight lines. In curves III. and IV., plotted for a rise in temperature of 50 degrees and 25 degrees respectively, we find in IV. that the amount of heat liberated with the 100 per cent. field is less than that liberated with the 75 per cent. field, and in curve III. the amount liberated with the 100 per cent. field is just equal to that liberated with the 75 per cent field. And when we come to consider curve No. II., plotted for

a rise of 75° C., we find that we have more heat liberated with the 100 per cent. field than with the 75 per cent. field; and similarly with curve I., for a rise of 100° centigrade. The above is apt to give rise to many questions as to the reason for the above results. Following we give what appears to us a satisfactory explanation. We will, however, leave others to judge whether it is or not, giving all the facts of the case to aid in the determination.

Part of the heat liberated from an armature leaves it by radiation, convection currents, etc., and part by radiation and conduc

tion. The poles or fields play a double part. In the first place, they prevent the escape of heat from the armature surface by destroying convection currents, and by interposing a warm surface very near to the armature; and secondly, they aid in carrying away a certain amount of heat by radiation and conduction. During the first portion of a run, or in starting up a dynamo, the fields being at the temperature of the room, and good conductors of heat aid materially in keeping down the temperature of the armature, by carrying off the heat produced, but as they become warmed up there is no longer the same difference of

temperature between the fields and the armature, and they rather prevent than aid in the liberation of heat. The fact that the fields become warm, and even hot, shows very plainly that they carry away a certain portion of heat from the armature. If we increase the external surface of the fields (that not next to the armature), why should they not carry away more heat from the armature? They would certainly radiate more heat and thus present a cooler surface to the armature, and the armature would in turn radiate more heat and its temperature be kept lower. Therefore if we have two pairs of fields, each pair covering an

equal amount of the armature, but one having a greater radiating surface than the other, we should expect that the armature would radiate more heat when the field having the greater radiating surface was used. If the above is true, and it seems reasonable, there is no reason why we should not be able to construct a field with a radiating surface sufficiently great so that, although it covered more of the armature, it would still keep the armature at a lower temperature than some other field which covered a smaller percentage of the armature. In the experiments made, the external surface of the 100% fields was considerably greater than

that of any of the other fields. And the above explanation will apply. It may be asked, why should the 100 per cent. field radiate more heat than the 75 per cent. field at 75° rise and 100° rise in temperature, when at 25° rise the amount of heat radiated with the 100 per cent. field is less than that radiated with the 75 per cent. field? The reason is that when the rise in temperature is greater, the amount of heat radiated per degree rise is greater, and the difference in the amount of heat radiated for a given rise in temperature is less with the 75 per cent. than with the 100 per cent. field.

When we examine the curves in Fig. 14, plotted for a peripheral velocity of 1000 ft. per min., we again find the same effect of difference of temperature. But in this case all of the curves are of single curvature and concave upward. As in the preceding case the amount of heat liberated at higher rises in temperature is greater for the 100 per cent. than for the 75 per cent. field. The curves are all very nearly parallel to each other and the greatest difference between the amount of heat liberated with the 75 and the 100 per cent. fields is 0.0004 watts per sq. inch per degree rise in temperature.

By an examination of the curves shown in Fig. 15 we find results similar to the above, but the curves have a double curvature, being at first concave downward, and changing gradually until they become concave upward. There is some uncertainty as to the curves plotted in Fig. 15. It was found quite difficult to obtain constant temperature when the cylinder was at rest. The temperature of the atmosphere would sometimes fall several degrees, and the cylinder being as rest, the time necessary for it and the fields to fall to the proper temperature was considerable. For this reason the figures are perhaps somewhat inaccurate.

If we compare now the curves in Figs. 12, 13, 14 and 15 we find that with a peripheral velocity of 3,000 ft. per minute we

have more heat liberated with the 75 per cent. than with the 100 per cent. field, while in all the other cases, at the higher temperatures, the opposite is true. The reason for this is that at the higher peripheral velocities the convection currents are greater and the field plays a smaller part in the amount of heat liberated. Whether the above explanation holds or not, there is of course the possibility of error in the experimental work. There must necessarily be some inaccuracy when the results depend upon the measurement of so many quantities. An error of two or three degrees would reverse many of the results obtained. If in .obtaining the temperature of the cylinder for the runs with the 100