i 4 4 SCIENCE PROGRESS. 



obtained, by which the number of useful lines has to be 

 multiplied in order to determine the total number of lines 

 passing through the field-magnet, and thence is calculated the 

 density of the lines in the field-magnet. It was, however, 

 pointed out that the value of the factor would vary according 

 as the armature was more or less saturated, and shortly after- 

 wards in the same year the magnetic leakage was dealt with 

 in an even more complete manner by Mr. Kapp in a paper read 

 before the Society of Telegraph Engineers * and also by Prof. 

 Forbes in the discussion which followed. By the aid of 

 their reasoning we are enabled to obtain a closely approxi- 

 mate figure for the number of leakage lines even when 

 dealing with a new and untried type of dynamo. Numer- 

 ous experimental determinations of the magnetic leakage in 

 various makes of dynamos have since been published ; 2 and 

 much ingenuity has been expended in designing " iron-clad " 

 dynamos, in which the field-magnet coils are enclosed in or 

 shielded by iron in order to reduce the leakage to a mini- 

 mum. It certainly is inconvenient to have our watches 

 magnetised and our measuring instruments affected by 

 dynamos at work near at hand ; but experience has shown 

 that it is easy to pay too high a figure in initial cost for the 

 advantages of a dynamo with a small external field, except, 

 of course, under special circumstances. One such circum- 

 stance is the use of large dynamos on board ship, where 

 they are apt to disturb the compasses. This effect has 

 manifested itself in a disagreeable manner in the case of 

 the ships of H.M. Navy. Not only have the compasses 

 been affected by dynamos, although situated at some dis- 

 tance away, but it is further stated that orders have been 

 issued forbidding the carrying of bayonets by marines when 

 on duty near working dynamos, since these iron arms be- 

 come magnetised by induction, and then subsequently when 

 brought near the compasses may deflect them. Quite 

 recently the authorities of the British Admiralty have in- 



1 Journal, vol. xv., No. 64, 1887. 



2 E.g., " Magnetic Leakage in Dynamos and Motors " (Ives), reprinted 

 in the Electrical Review, 22nd and 29th Jan., 1892, and " Magnetic Data of 

 Sprague Motor'' (Parshall), Electrical Engineer, 13th June, 1890; also 

 (Puffer), Electrical Review, 15th April, 1892 



