128 ; 
more sensitive than the ordinary form of D’Arsonval, and at the same 
time, make use of it in places where there was a liability to local changes 
in the earth’s field. 
But in this we were disappointed, for the reason that it was found im- 
possible to make a deflecting coil free from magnetic impurities in both 
the insulation of the wire and the wire itself. 
This magnetic property of the deflecting coil, it is easily seen, intro- 
duced a moment opposite to that which tended to produce a deflection of 
the coil, and therefore greatly reduced the sensibility of the instrument. 
This last difficulty would be entirely climinated if the controlling field 
were uniform within the limits and in the direction of the motion of the 
deflecting coil. 
This has, I believe, been accomplished in the instrument I will now 
describe: 
The field magnet is an ironclad electro-magnet of cylindrical form (Fig. 
1), the enveloping shell (IX) being simply a continuation of the core of the 
electro-magnet M, the only opening for the magnetic field being an annular 
slot 4%” wide, 154” outside diameter and of depth 14”. In this slot the 
deflecting coil (G) is suspended by means of its two terminals (R, R). 
This form of field magnet will give us a uniform field in the direction of 
the slot, provided the two cylinders, forming the slot, have their axes 
parallel, (not necessarily coinciding). 
Still another provision was made to guard against the non-uniformity 
of the field by multiplying the motion between the deflecting coil and the 
mirror so that a very small displacement of the coil, and consequently in- 
sensible change of field in which it hangs, would produce a readable deflec- 
tion of the mirror. 
This multiplication is accomplished by the peculiar suspending mech- 
anism. 
Between the two pillars (A, A) there are suspended two light brass 
pulleys (P, P) by means of the piano wires (W, W) (diameter .01”). 
These and the wires are insulated from each other by the piece (C), the 
faces of the pulleys are directly over the field-gap, and wrapped around 
them are the two terminals (R, R) of the deflecting coil (G), so that an 
electric circuit can be established between the pillars (A, A) by way of the 
deflecting coil. 
y-. 
