1891.] on Electric and Magnetic Screening. 349 



of the Navy. It gives an interesting illustration of magnetic screen- 

 ing effect by the case of a belt of iron, 1 foot thick, 5 feet high, and 

 10 feet in internal diameter, with roof and floor of comparatively thin 

 iron. Captain Creak informs me that the average horizontal com- 

 ponent of the magnetic directing force on the compass in the centre 

 of this conning-tower is only about one-fifth of that of the undisturbed 

 terrestrial magnetism. 



An evil practice, against which careful theoretical and practical 

 warnings were published two or three years ago,* and which is now 

 nearly, though, I believe, not at this moment quite thoroughly, 

 stopped, of what is called single wiring in the electric lighting of 

 ships, has been fallaciously defended by various bad reasons, among 

 them an erroneous argument that the ship's iron produced a sufficient 

 screening effect against disturbance of the ship's compasses, by the 

 electric light currents, when that plan of wiring is adopted. The 

 argument would be good for a ship 60 feet broad and 30 feet deep, if 

 the deck and hull were of iron 3 feet thick. As it is, mathematical 

 calculation shows that the screening effect is quite small in comparison 

 with what the disturbance of the compass would be if the ship and 

 her decks were all of wood. Actual observation, on ships electrically 

 lighted on the single wire system by some of the best electrical 

 engineers in the world, has shown, in many cases, disturbance of the 

 compass of from 3 degrees to 7 degrees, produced by throwing off and on 

 the groups of lights in various parts of the ship, which are thrown on 

 and off habitually in the evenings and nights, in ordinary and neces- 

 sary practice of sea-going passenger ships. When the facts become 

 known to shipowners, single wiring will never again be admitted at 

 sea unless the alternating current system of electric lighting is again 

 adopted. But, although this system was largely used when electric 

 lighting was first introduced into ships, the economy and other advan- 

 tages of the direct-current system are so great that no one would 

 think of using the alternate system for the trivial economy, if any 

 economy there is, in the single wire, as compared with the double 

 insulated wire system. 



An interesting illustration of a case in which iron, of any thick- 

 ness, however great, produces no screening effect on an electric current, 

 steady or alternating, is shown by the accompanying diagram, Fig. 7, 

 which represents in section an elecT;ric current along the axis of a 

 circular iron tube, completely surrounding it. Whether the tube be 

 long or short, it exercises no screening effect whatever. A single 

 circular iron ring, supported in the air, with its plane perpendicular 

 to the length of a straight conductor conveying an electric current, 

 produces absolutely no disturbance of the circular endless lines of 

 magnetic force which surround the wire ; neither does any piece of 



* See ' The Electrician,' vol. xxiii. p. 87. Paper read before the Institution 

 of Electrical Engineers, by Sir "William Thomson, "On the Security against 

 Disturbance of Ships' Compasses by Electric Lighting Appliances." 



