N. D. C. Hodges—Absolute Galvanometer, 475 
Art. LXII.—On a new Absolute Galvanometer ; by N. D. C. 
Hope@Es. 
In the ordinary form of galvanometer the current is measured 
by the ratio of the force it exerts on the needle to the directive 
force of the earth, the ratio being determined by a measure- 
ment of the angle ‘of deflection. 
The moment of the force with which a unit weeds acts on 
the needle may be expressed in a series of the for. 
G, g, sin? + G, g, sin? Q,'(9) + ete. 
re G,, G, are constants depending on the dimensions of 
sy ao and g,,g, on those of the Or 0. apparatus, coil 
increasing the number of coils and suitably placing them, the 
magnetic field may be rendered more uniform. 
In reading the deflection either a divided circle or a telescope 
and scale may be used. With the divided circle the deflection 
may be as great as 45°, but not more, or else the instrument 
would not be sensitive to changes in the current. The use 
of telescope and scale necessitates much aie deflections. 
To regulate the strength of the current shunts of small resist- 
ance often have to be used ; and the ep of the current 
through the instrument is rendered dou 
If, instead of placing the plane of the ae parallel with the 
magnetic meridian, they are placed perpendicular to it, the 
sum of the force of the current poo of the directive force of 
the earth would —— the m sana 
The formula ee (+5) 
expresses the relation between the time of vibration of a hori- 
zontal, swinging magnet, its moment of inertia K, its magnetic 
moment M, and the horizontal component of the earth’s mag- 
netic force 
If this time, the time of vibration without the current, is 
first taken and then the time ¢, with the current, we get the 
relation between oa ive, 
x M(T+F) (1+ 9) 
¢ T4+F ee 
et EF 
é 
