TransactionsPublished under the care of the General Secretary and the Treasurer . nstitute. An impression seems to be abroad tliat the knoAvn laws of electro-magnetism do not suffice to enable the pull of the solenoid upon aniron plunger within it to be calculated, or at least that there issome obscure difficulty that prevents the design of such plungerelectromagnets from being readily predetermined, so that whenconstructed they shall fulfil any prescribed specification as to therequired pull and range of travel. The author has, however, notfound much trouble in working out a set of rules for
TransactionsPublished under the care of the General Secretary and the Treasurer . nstitute. An impression seems to be abroad tliat the knoAvn laws of electro-magnetism do not suffice to enable the pull of the solenoid upon aniron plunger within it to be calculated, or at least that there issome obscure difficulty that prevents the design of such plungerelectromagnets from being readily predetermined, so that whenconstructed they shall fulfil any prescribed specification as to therequired pull and range of travel. The author has, however, notfound much trouble in working out a set of rules for design whichyield results of an accuracy quite comparable with that attainedby the customary rules for the designing of electromagnets fordynamos and motors. It is believed that these will be welcomed byengineers interested in this branch of construction. Hence a briefaccount of them and of the principles on which they are est<ablishedis here given. To concentrate attention on the essentials of the problem con-sider a magnetic circuit constituted as shown in Fig, 1, where a. Fig. 1. cylindrical rod of soft iron is supposed to be bent so that its endsenter a tubular bobbin carrying a magnetizing coil. Let the lengthof the gap between the ends of the rod be called z inches, and letthe excitation be such that the flux-density along the gap, whichwill be practically uniform, be B lines per square inch. Let each [542] THOMPSON.• PLUNGER ELECTROMAGNETS. 543 ^i the end faces of the iron have an area of A square inches. Thenit is known that the pull along the gap z, tending to bring the twoend faces of the iron together, is expressed by the formula P = 5-^/72,134,000; (1) where P is the pull in pounds weight. It is also known that theflux-density B in the gap is related to the excitation according tothe formula BX2 X = /iS; (2) where IS stands for the number of ampere turns of excitation thatare actually expended in driving the magnetism across the gap, is the gap
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Keywords: ., bookcentury1900, bookdecade1900, bookidtr, booksubjectelectricity
