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C. F. Brackett—New form of Galvanometer. 395 
easily illustrated by touching the electrode upon which the glow 
appears, and then removing the hand. The shadow, which 
at first covers almost the whole surface of the ball, rapidly 
diminishes as the tension rises. The size of the shadow further 
depends on the distance of the interposed object from the 
glowing electrode, though the relations of size and distance 
could not be very definitely determined. When the point 
and ball were used as terminals, the shadow appeared greatest 
when the interposed object was near either electrode and least 
When it was half way between them. When the two balls 
were used and both glows established at once, and generally 
when the discharge opposite the glow came from several points, 
the shadow decreased regularly as the interposed object was 
further removed. 
It would not be proper for us to omit mention of the fact 
that Professor C. A. Young has since informed us that he 
noticed the existence of the negative shadow several years ago. 
J. C. Green School of Science, Princeton, N. J. 
Art. LI. — Note on a New Form of Galvanometer for Pow- 
hag Currents; by Professor C. F. Brackett, College of 
rsey. 
THE very powerful currents produced by large dynamo- 
machines are not easily estimated by the appliances usually 
found in the physical laboratory. ; 
The various forms of the electro-dynamometer, the cosine 
galvanometer and some other special forms of apparatus may of 
_ Course be employed with satisfactory results. None of these 
Instruments being in the collection at Princeton, it was deter- 
mined to construct a galvanometer which should obey the law 
of tangents and yet not be so large as to be unwieldy. In 
order to this, recourse was had to the differential principle. 
The construction is as follows : 
ing, with hard solder, between the ends on one side of the cut, 
4 piece of metal having the same cross section as that of the 
hoops, and of suitable length. At several other points are 
Inserted between the hoops pieces of hard rubber of proper 
thickness, which serve to keep them truly concentric. 
The three ends of the system thus arranged and set upright 
On a proper base board are joined to binding screws. 
It will be seen that the differential action on a needle placed » 
D.. @éthe center, or on the axis of the hoops passing through their 
