SECTIONAL TRANSACTIONS.— G. 363 



Two fatigue-testing machines are described which have been specially 

 developed to study (a) the various stages of the crack-spreading process ; 



(b) the modifications caused by the contact pressure of loaded rollers ; 



(c) the influence of periodically varying the limits of successive cycles of 

 alternating stress. 



Attention is drawn to certain constantly recurring features of service 

 breakages which have been successfully reproduced in the laboratory tests. 

 It is shown that some of these are related to the direction of rotation. 

 Various methods of crack detection are illustrated and compared, and the 

 possibilities of fracture study as an aid to diagnosing causes of failure are 

 discussed. Some examples of corrosion fatigue are included in which the 

 corroding agent is a mixture of wet steam and air. 



A collection of the author's photographs will be placed on view previous 

 to the reading of the paper. 



Dr. H. Cotton and Mr. F. A. Hough. — The pulling into step of a 

 synchronous-induction motor. 



The investigation of the transition from induction motor to synchronous 

 motor running involves the solution of a differential equation necessitating 

 a very laborious step process, in consequence of which the mathematical 

 solution of the problem has been confined to a few special cases. In 

 America solutions have been made by means of the integraph, a machine 

 which is not available in this country, and which also has the disadvantage 

 that it is a purely mechanical contrivance giving the operator no insight 

 into the real nature of the problem under investigation. 



The present research deals with a solution obtained from the examination 

 of the motion of a mechanical model whose equation of motion is the same 

 as that of the synchronous-induction motor. Cinematograph films were 

 taken while the model was in motion and the angular position at any instant 

 determined from these. The advantages of the method are : (a) the con- 

 stants of the model can be altered at will, so that the performance of any 

 motor can be imitated ; (b) the motion to be examined is a slow slip motion 

 and not the high speed of an actual motor, on which the slip motion required 

 is superposed ; (c) the model can be made very cheaply ; (d) other pheno- 

 mena — e.g. re-synchronising after falling out of step — can be investigated in 

 addition to the pulling into step. 



There were many difficulties to be overcome before a suitable model was 

 developed, and these are described in the complete paper. 



Dr. T. F. Wall. — The economical control of the speed and power factor of 

 three-phase induction motors. 



The problem of the economical control of the speed and power factor 

 of three-phase induction motors by cascade connection with alternating 

 current commutator machinery has become one of rapidly increasing 

 industrial importance. The investigation of this problem is greatly 

 facilitated by the use of the complex quantity method of treatment. In the 

 first part of the paper some of the characteristics of the standard equation 

 for the circle diagram are considered, when this equation is expressed in 

 terms of complex quantities with the slip as the variable. In the next 

 part of the paper it is shown how the most favourable circle diagram for 

 the purpose in view may be chosen at will and the motor operated on a 

 shunt characteristic with the same circle diagram and having any desired 

 no-load speed. In the last part of the paper the conditions are established 



