646 TRANSACTIONS OP SECTION G. 



the desired ' correcting ' voltage. Curves are given showing how the correcting 

 voltage varies with the load. 



If for any reason the commutating pole tends to become too weak, owing, 

 let us say, to the saturation of the iron of the pole, the induction of the 

 armature coils carries the current forward to brush A, and tends to increase 

 automatically the excitation of the commutating pole. 



A 12 5 K.W. generator fitted with the device will be exhibited. 



A .500 K.W. 500 volt rotary converter, built by the British Westinghouse 

 Co., was fitted with the double brush gear, and it was found that with 

 3,000 amperes flowing the commutation was sparkless. The device is being 

 at present installed on a 1,500 K.W. rotary converter, on which the com- 

 mutating conditions would otherwise be rather difficult. 



3. Electric Oscillations in Coupled Circuits — a Class of Particular Cases. 

 By W. EccLES, D.Sc, and A. J. Makower, M.A.'^ 



If three capacity-inductance branches are connected in parallel and vibra- 

 tions are started in the system, these vibrations consist, in general, of two 

 superposed simple harmonic vibrations of distinct frequencies. It is shown 

 that if two of the branches are in tune a class of problem arises which has a 

 very simple solution. Incidentally new formulae are ari-ived at for calculating 

 the frequency of the two oscillations in a pair of circuits coupled by inductance 

 and capacity. It is shown also that for a single frequency to exist all three 

 branches must be tuned to the same frequency. All the formulae developed in 

 the paper are verified by experiments extending over a wide range of values 

 of inductance and capacity. 



4. The Capacity of Aerials of the Umbrella Type. 

 By Professor G. W. O. Howe, D.Sc.^ 



In a paper read before the British Association at Sydney last year the 

 author developed a method of calculating the capacity of radio-telegraphic 

 antennas. In addition to describing the method in general, curves and formulae 

 were given so that the capacity of aerials of standard types could be deter- 

 mined in a few minutes. The umbrella type, however, was not specially con- 

 sidered, and it has since been suggested "to the author that the usefulness of 

 the paper would be considerably increased if curves and formulae could be 

 given for aerials of this type. The method is briefly as follows : The whole 

 aerial is assumed to have a uniformly distributed charge, and the average 

 potential of the whole aerial under this fictitious condition is then calculated. 

 It is assumed that if the total charge, while remaining unchanged in quantity, 

 be allowed to have its own natural distribution, it will assume a uniform 

 potential approximately equal to this fictitious average potential. The 

 proximity of the earth is taken into account by the method of images. Tables 

 and curves are given for aerials with from two to six ribs and for various 

 angles between the ribs and the vertical. With these curves and those given in 

 the original paper, each of the nine component potentials of any given aerial of 

 the umbrella type can be read off and the resultant average potential deter- 

 mined. The method is then applied to two practical examples, one a simple 

 aerial with six single-wire ribs, and the other a more complicated case in which 

 each of the five ribs consists of a four-wire cage, the size of the wire being 

 different from that used for the central vertical wire. 



Tests on actual aerials have shown that the values of the capacity as cal- 

 culated by the author's method agree with the measured values within the 

 errors of observation and of estimation as to the allowance to be made for 

 connecting wires, &c. 



' >See T/te Electrician, vol. Ixxv., p. 905. 

 = Ibid., p. 870. 



