Effect of Electrical Convection. 35 



potential within as great limits as possible. To do this, the 

 same method as that previously used by the author, and first 

 introduced by M. Cremieu, was adopted, namely, to measure 

 the current induced in a coil when the charge on a rapidly 

 rotating disk close to it is suddenly reversed. 



The great difficulty encountered in the experiments of last 

 year was the impossibility of shielding the needle of the 

 sensitive galvanometer employed to detect this current from 

 the disturbing magnetic effects of the electric circuits in the 

 vicinity of the laboratory, although the experiments were 

 conducted at night after the electric cars had ceased running. 

 Through the kindness of Prof. Ames, Director of the Physical 

 Laboratory of the Johns Hopkins University, I was enabled 

 to move the entire apparatus to the country. The apparatus 

 was therefore set up at McDonogh School, twelve miles from 

 the city of Baltimore and two miles from the nearest electric 

 oar-line. Experiment showed that this car-line w T as too 

 distant to affect the galvanometer. Electric power for 

 running the various motors was furnished by the school. 



The room first put at my disposal was a large garret. The 

 apparatus, was set up here, and considerable time was spent 

 in vain attempts to mount the galvanometer so as to be free 

 from mechanical jarring. This was finally given up as im- 

 possible, as it was found that the wind caused the whole 

 building to rock considerably. The apparatus was then 

 taken down and set up once more in a large basement room 

 of another building. This room was 14'5 x 19'5 m. in size 

 .and had a cement floor, which was so solid that when the 

 galvanometer was mounted on a stout table the needle was 

 entirely free from mechanical jarring. 



The various parts of the apparatus and their general 

 arrangement were essentially the same as employed last year. 

 There were, however, a few changes made which will be 

 briefly noticed. 



The disk apparatus was changed only in a minor point, 

 which, however, obviated a source of great inconvenience. 

 This change was the mounting of the brushes making contact 

 with the surfaces of the disks in such a way that the insula- 

 tion of the cores could be cleaned without taking the apparatus 

 apart. Special care was taken to clean this insulation 

 thoroughly before each set of readings. 



A new needle for the galvanometer was made having a 

 greater sensibility than the old one, and the ground-glass 

 scale was placed two metres from the galvanometer. With 

 the new needle and scale thus arranged it was possible to get 

 four times the sensibility previously secured. But it was 



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