80 Dr. E. Obach on an Improved Construction 
It will be noticed that the value of the interval only changes 
at those places ■where the tangent is equal to a whole figure, 
thus making a mistake in reading less likely. Looking at 
this scale no gaps are conspicuous, and the divisions are 
everywhere pretty evenly distributed. Tangent-scales have 
been employed by Joule, Sir William Thomson, and others; 
but the one now described seems -well to satisfy all the re- 
quirements. 
Inclination-scale. — This scale, engraved on a vertical qua- 
drant divided into degrees, can accurately be read to one tenth 
by means of a vernier. The zero division was formerly that 
to which the index pointed when the ring was horizontal. In 
this case the tangent of the deflections had to be divided by 
the sine of the angles. For convenience' sake, the places 
were specially marked on the scale at which the sines corre- 
sponded to whole figures. The new inclination-scale has the 
zero at the vertical or normal position of the ring; and instead 
of the sines, the secants are specially marked which are repre- 
sented by whole figures. With these secants the tangents of 
the deflections must be multiplied; and they can therefore be 
termed multiplying powers, analogous to the multiplying 
power of shunts. The instruments intended only for the 
measurement of current-strength have the quadrant bearing 
the secant-scale fixed outside the ring, whilst the others, mea- 
suring also electromotive force, have it situated between the 
needle-box and the ring, where it is better protected from injury. 
If the deflections of the needle are read on the tangent- 
scale and the positions of the ring on the secant-scale, the aid 
of trigonometrical tables may be entirely dispensed with, as 
the product of the two figures represents the quantity to be 
measured, irrespective of a constant. 
The Solid Rixg a^td the Coil. 
If the galvanometer has to serve only for the measurement 
of currents, the gun-metal ring is of a rectangular cross 
section; but if it is at the same time destined to measure dif- 
ference of potential, the cross section is V-shaped, the groove 
being filled with numerous turns of G.S. wire. If the num- 
ber of convolutions is known, and if a simple relation exists 
between that number and the resistance of the wire, a great 
advantage may be derived therefrom. For instance, if there 
are one thousand convolutions on the coil, offering a resistance 
of exactly one thousand ohms, the current due to the difference 
of potential of one volt at the ends of the coil would produce 
the same deflection of the needle as the current of one ampere 
flowing through the solid metal ring. That this must be so 
