Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . er cent (.0021) for each degree F. for eachohm, and iron wire more than one-quarter of 1 per cent(.002517). Prob. 115: The resistance of the field magnets of dynamo is 55ohms at 70° F.; after a ten-hour run the temperature of a thermome-ter placed against them, for a short time, is 94°. (a) What is theirresistance at this temperature? (b) What would be the resistance at40° By Formula (100) Rx= (R X T X F.° rise) + R = (55 X .002156 X 24) + 55 = ohms (a).R = 55 ohms, T for
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . er cent (.0021) for each degree F. for eachohm, and iron wire more than one-quarter of 1 per cent(.002517). Prob. 115: The resistance of the field magnets of dynamo is 55ohms at 70° F.; after a ten-hour run the temperature of a thermome-ter placed against them, for a short time, is 94°. (a) What is theirresistance at this temperature? (b) What would be the resistance at40° By Formula (100) Rx= (R X T X F.° rise) + R = (55 X .002156 X 24) + 55 = ohms (a).R = 55 ohms, T for copper wire = .002156, F.° =94 - 70 = 24° Formula (101) R,= R - (R X T X F.° fall) = 55-(55 X .002156 R = 55 ohms, T=. 263. Fuses and Cut-Outs.—When a piece of copper andlead wire of the same size are connected in series and a currentpassed through them so that their temperature is increased, thelead will melt when a temperature of 612° F. is attained, while ,= R- (RX fall) = 55-(5 X 30) = = ohms (b). 002156, = 70° _ 40° = = 30° fall. ELECTRICAL DEVELOPMENT OF HEAT 273. it will require a temperature of 1996° F. to melt the containing a small percentage of tin is used for electricfuses on account ofthe low tempera-ture at which itmelts. A fuse con-sists of such a lead-en wire or stripwhich is inserted inseries with the cir-cuit it is desired toprotect, and de-signed so as to melt,and thus automati-cally open the cir-cuit when the cur-rent through it becomes excessive. The fuse is mounted ona porcelain block and the appliance termed a cut-out. Thecanning capacity of a fuse depends upon its cross-section,and fuse wire is generally rated to be of so many amperescapacity, meaning that it will carry this current withoutmelting or blowing, as it is termed, and melt on a slightincrease in current above its capacity. The function of afuse, therefore, is to open the circuit before the temperaturerise due to an excessive current from any
Size: 2031px × 1230px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyo, bookyear1901
