54 
MESSKS. C4. F. C. SEARLE AND T. D. BEDFORD 
upper pair of colls and the loAver end is at the centre of the lower pair of coils. The 
motion of the magnets is observed hy the aid of a lamp and scale. 
The galvanometer is very sensitive, and thus measurements of B for quite thin iron 
wires can he made with a comparatively small number of turns of secondary winding ; 
this is often a point of some convenience. 
We have found the instrument very efficient. The time of vibration, 12 seconds, 
is long enough to enable the ballistic throws to be read with ease. Since the needle 
is practically astatic, the zero depends only on the action of the bronze wire and not 
upon the earth’s magnetic field. Tlie result is that the zero is remarkably constant, 
often not changing l)y more than one-tentli millim. during several hours. The onlv 
disadvantage is tliat there Is so little damping that, to bring the needle to rest in 
any reasonable time, it is necessary to use a coil of wire placed near the galvano¬ 
meter in conjunction with a Leclanche cell and a tapping key. A little practice 
enables the observer to bring the s]:)ot quickly to rest. 
The restoring couple varying as the angle of deflexion instead of as its sine, the 
time Integral of a transient current is proportional to tlie angle of throw instead of 
the sine of half that angle. We verified l)y experiment that this law is accurately 
obeyed. 
With rise of temperature the magnetic moment of a magnet diminishes, and we 
conse([uently found that with the same resistance in circuit the throw due to a given 
change of induction was rather less on a hot than on a cool day. 
The logarithmic decrement depends upon the resistance in circuit with the galvan¬ 
ometer. But in every case the resistance of that circuit was kept constant during 
a set of observations, and thus all error due to this cause was avoided. 
When we desired to draw a cyclic B—H curve for a specimen of iron, we 
practically followed the method described hy Professor J. A. Ewtng^. 
The Electro-dynamometer. 
§ 27. The electro-dynamometer employed iir the later experiments has a pair of fixed 
coils, each formed of 250 turns of No. 20 B. W.G. cotton-covered wire wound on ebonite 
bobbins. The mean radius of the coils is about 5 centims., and their resistance in 
series about 4 ohms. In order to avoid induced currents there were no large pieces 
of metal near tlie coils. In the central space hangs the suspended coil. This 
consists of 190 turns of No. 40 B.W.G. silk-covered wire. The mean radius of the 
coil is 1’7 centims. and its resistance about 28 ohms. The coil is attached along a 
diameter to a stiff’ brass wire, whose upper end carries a mirror. The mirror is placed 
so far above the centre of the coils that the beam of light from the lamp passes 
above the outside of the fixed coils. The moving coil is suspended by a johosphor- 
bronze wire diameter, and about 4 centims. long. The other connexion 
* “ j\[agiietic Induction in Iron and other Metals,” .Ird Ed. Revd., § 192. 
