. The street railway review . r, of general interest. It is quite important in many cases, particularly where alternat-ing current generators are worked in multiple, that not onlyshould the number of revolutions per unit of time be as nearlyconstant as possible but also that the variation in speed duringone revolution should be within a narrow limit. In the case ofan alternator the variation in speed during a single revolution isusually given in terms of the angle by which the center of thecrank pin diverges from the position it would have were thespeed constant. However, this angle is not giv
. The street railway review . r, of general interest. It is quite important in many cases, particularly where alternat-ing current generators are worked in multiple, that not onlyshould the number of revolutions per unit of time be as nearlyconstant as possible but also that the variation in speed duringone revolution should be within a narrow limit. In the case ofan alternator the variation in speed during a single revolution isusually given in terms of the angle by which the center of thecrank pin diverges from the position it would have were thespeed constant. However, this angle is not given in degreesof arc, but in degrees of phase departure, which may be calledpole-degrees, a pole degree being in each case the angle cor-responding to two poles on the machine divided into ,^60 on a 40-pole generator i degree of arc is equal to 20degrees of phase departure or 20 pole-degrees. After deriving curves showing the crank displacement for cer-tain engines having the high and low pressure cranks go° aparr,. il was shown that the displacement coidd be greatly recluccil bychoosing a difTerent crank angle. We reproduce two diagrams from .\lr, Astronis p;i])er. Fig. 1shows the curves of tangential crank cfTort of a 44 in. and 86 60 in. vertical cross com])ound condensing engine developing^■fy>, , 6,070 and 6,8jo h. p., respectively, at a speed of 75r. p. m. The cranks are set at 90°, the high pressure engineleading. Fig. 2 shows corresponding curves of crank efiforl under thesame conditio;is of load, but with the low pressure crank. 120in • ,.• _. ., ■■.• .. ..•..■ In both diagrams the ordinates show the tangential crank elTortand the abscissae tile position of the crank pin in its path, theliuir horizontal lines corresponding to the four uniform loads. In conclusion, the author says that by using the gives the most uniform rotative efifect, the displacementof crank pin has been found to get reduced by 15
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