Instrument for Measuring Magnetic Fields. 189 



Fig. 4 (Plate III.) shows the fields round Mr. Kapp's 

 8-pole machine. They are noticeably small. 



Fig. 2 (Plate IV.) shows the effect of edges, corners, &c. on 

 the strength of field. On the flat surface of the pole-piece 

 the field was about 600 (varying between 517 and 690), 

 on the edges increased to about 1000, whilst on the corners 

 it reached a strength of over 1100 C.G.S. lines per centimetre. 



Fig. 3 (Plate IV.) shows the deformation of the stray field 

 produced by the armature reactions. The measurements were 

 made on an Elwell- Parker Motor. The strength of field on 

 the trailing edge was about 460, whilst that on the leading 

 edge was about 500. 



Fig. 6 (Plate III.) shows various measurements made on 

 the Grulcher Dynamo ; Fig. 7 (same Plate) gives the field 

 near one of Mr. Kapp's Dynamos ; Fig. 4 (Plate IV.) shows 

 the fields at two positions of a Laing, Wharton, and Down 

 shielded dynamo. 



Some curious effects of armature reactions are noticeable 

 on the Thomson-Houston Dynamo (fig. 5, Plate III.). As 

 the bars in this machine act as a yoke, the result is due 

 to combined magnetic leakage and armature reaction. 



By the kindness of Mr. Harrison we were enabled to make 

 some experiments on a watch, previously unmagnetized, which 

 he lent us. We found that a field of about 10 had no appre- 

 ciable effect on its rate of going, but that after being subjected 

 to a field of about 40 it lost about 8 minutes per day ; and 

 even after being demagnetized in an alternating field it still 

 continued very erratic in its actions. Of the dynamos whose 

 fields we have measured, with the exception of the Thomson- 

 Houston,, Ship's Dynamo (Laing, Wharton, and Down), and 

 Mr, Kapp's large Multipolar, it would not be safe to go nearer 

 than about 2 feet*, Moreover, with a watch with a steel 

 balance-wheel (the one experimented upon had a brass one), 

 even greater precautions might have to be observed. 



Finally, we wish to record our thanks to Mr. Harrison for 

 allowing us to experiment on his watch ; to Mr. Barton for 

 his assistance in constructing the instrument ; and to Messrs. 

 Crompton, Kapp, Laing, Wharton, and Down, the Gulcher 

 Company, the Electric Construction Corporation, and Major- 

 General Festing, for permission to experiment on their various 

 dynamos, and also to publish the results. 



* One could not safely go within three feet of the Elwell-Parker 

 Continuous Current Transformer. 



