334 Transactions. — Miscellaneous. 



the resistance amounted to 40 ohms, after they had been kept 

 in use for some months, when no precautions had been taken 

 to prevent evaporation. But precisely similar cells in which 

 the sawdust was thoroughly moist had no higher resistance 

 than 8 ohms. 



The effect of the movements of the ship on the galvano- 

 meter needles was overcome by the invention of Thomson's 

 marine galvanometer. This is a modification again of that 

 gentleman's mirror galvanometer. The latter instrument is 

 so sensitive that it will indicate a current if one simply 

 presses separate fingers on the two terminals. I have also 

 obtained a very sensible deflection on an ordinary instrument 

 of this description by working a small frictional machine for 

 a few minutes and collecting electricity from the air of the 

 room. In this case, although the electricity generated is at a 

 high potential, the current obtained from the air is naturally 

 an extremely feeble one, but the mirror galvanometer is suffi- 

 ciently sensitive to indicate a current flowing for some 

 seconds, or even a minute or two. Everything possible is 

 done to make this instrument sensitive. It is composed of 

 coils consisting of thousands of turns of copper wire, and 

 the magnetic intensity is reduced to a minimum by having 

 several magnets, one-half the number having their north poles 

 immediately over the south poles of the other half. But 

 what more than anything else makes the instrument ex- 

 tremely sensitive is the employment of a small mirror at- 

 tached to the magnets to reflect a beam of light on to a 

 scale. This arrangement gives virtually a pointer of a foot 

 or two in length, but absolutely without weight. 



A similar course is followed with the marine galvanometer. 

 The galvanic needles are rendered astatic in the same way ; 

 they are surrounded by coils containing thousands of turns of 

 wire, and the deflection is shown by the movement of a beam 

 of light. In the land instrument it is sufiicient to suspend 

 the mirror with the magnetic needles attached by a single 

 fibre, and they are allowed to hang and swing freely, because 

 the instrument can be kept in a vertical position. In the 

 marine instrument the vertical position cannot be maintained 

 on account of the ship pitching and rolling, and it is therefore 

 necessary to use several fibres, and to attach them at the 

 bottom as well as at the top. In this way a much stronger 

 current is necessary to deflect the needles. Again, it is neces- 

 sary to minimise the efl'ect of the earth's and the ship's 

 magnetism on the instrument, otherwise ib would be more 

 sensitive with the ship in one position than another, and it 

 would fail to indicate the principal thing required of it — the 

 real condition of the cable. For this purpose several power- 

 ful magnets are placed in the instrument, and the whole is 



