The Montgomery advertiser., 1901-07-26

sum of the separate terms represents tha complete reluctance. A typical case. ike that shownsin' Fis. II would require i've terms, the two cast iron poie pieces N and s. the, two wrought iron field cores F, the magnetic yoke Y. the armature core c. and the two air raps betwen polar face and arrna- ture core. Air is such poor conductor of magnetism that by far the greater portion of the winding on field mag net is for this portion of the reluctance A properly designed dynamo will allow about 75 per cent. of the entire m. m. f. to care for the air gap; this means that the rest of the magnet circuit must be of short length and of liberal cross section.
 In dynamo calculations the number of lines of force Is assumed to comply with the formula given earlier in this article, and the formula last given is reversed so as to tell what number of anlpere turns Is necessary to produce that strength of field. expressed in this convenient form and transferred to English units-inches in length and square inches in area-CS equals 3l4xNx total reluctance.
 Various other important factors need to be considered by the electrical en- giueer, involving the leakage of mag netism, energy allowable for field ex- citation, conditions involving sparkling at the brushes. method of regulation, etc. that, of course, could not be ex- plained outside of extended technical treatises. Prominent characteristics will be noted in connection with the articles explaining actual machinery.
 In general the qualifications or adap. tabiity of a dynamo to particular uses depend upon the relative values of the strength of field and number of arma- ture wires, or, using the first formula upon the relation of N and z. In. arc light dynamos N is comparatively small, but by use or a limited quantity of iron in the field magnets these are highly saturated:' z Is comparatively large. In dynamos for supply of incan- descent lamps and motors the propor- tions are reversed; z Is small. N5iarge. and the iron in such quantity as to be far from saturation.
 A further distinction in the practi- cal uses of dynamos Is made in the method of exciting the field magnets. If all the current from the armature flows around the fields and in a single path in the external circuit the ma GHtIe tb COtlHCCtC4 oNAtOo Ahu sss adapted for operating arc lamps also series-everything lis in series. See figure IS. Such would be called a constant current" dynamo. If only a portion of the urrent is to be used to excite the field flowing through a approximately larger number of turns of finer wire. the machine Is connected 'shunt" whether the external circuit is open or closed, the fields will always be magnetized; such machine Is useful for operating incandescent lamps: regu- lation must be effected by inserting a variable resistance called .'rteostat' in the field coil circuit. A combina- ton of the two methods. ehown in figure 20, is denoted as compound. ' Tn order to secure an available con stAnt potential or voltage the field ma gnets should be slightly. increased in strength se the current increases; the few series turns <of wire add just enough lines of force to those produced by the shunt to supply this need. there by effecting automatic regulation. mak- ing suct machine the most common for supply of incandescent lamps and motors.
 Some definite data of actual ma chimes will be given in subsequent articles.
 (Copyright, l90o.)