1903.] The" Hunting" of Alternating -Current Machines. 235 



Fig. 28. Gold leaf on glass, exposed to mercury vapour, and then heated sufficiently 



to drive off the mercury. 

 Transmitted light, green screen. 

 Objective 12 mm. N.A. 0'65. Magnification x 440. 

 The dark patches were deep green, and the clear parts between them 



were colourless. 



Fig. 29. Silver film on glass, annealed by heating at 350. 

 Transmitted light with green screen. 

 Objective 12 mm. N.A. 0'65. Magnification x 410. 



Fig. 30. The same by dark ground illumination. 

 (Oblique transmitted light). 

 Objective 16 mm. N.A. 0'3. Magnification x 440. 



" The ' Hunting ' of Alternating-Current Machines." By BERTRAM 

 HOPKINSON, M.A. Communicated by Professor J. A. EWING, 

 F.R.S. Received June 16, Read June 18, 1903. 



Many years ago the late Dr. John Hopkinson showed that if a pair 

 of alternating-current dynamos, A and B, mechanically separate but 

 connected electrically in parallel, be running steadily on a constant 

 load and with a constant driving power, and if the steady motion 

 be slightly disturbed, say by momentarily retarding A, then A will 

 do less and B more than its share of the work, with the result that 

 there will be a balance of force tending to accelerate A and to retard 

 B and so to restore the state of steady motion. In other words the 

 two machines tend to keep in step. Similar considerations apply to a 

 synchronous alternating-current motor worked from supply mains it 

 tends to keep in step with the generators supplying it. 



It has been found in practice that as a general rule the paralleled 

 alternators do keep in step, but in a not inconsiderable number of 

 cases great trouble has been caused by a tendency in the machines 

 to develop gradually increasing oscillations about the state of steady 

 motion in which they are in step with one another. This oscil- 

 lation or "hunting" leads to violent cross magnetising currents, 

 and sometimes the machines drop out of step altogether. This 

 phenomenon has received a great deal of attention from the practical 

 side, the object being of course to put an end to it. This 

 experimental study has resulted in empirical rules as to fly-wheel 

 effect, and in the various damping devices or " Amortisseurs " which 

 are now largely used on alternating-current machinery and generally 

 give satisfactory parallel running. 



Theoretical treatment of hunting has been confined (so far as I am 



