Fic. 1.—Arrangement for Six Circuits. 
REPORT— 1896. 
+20 
Scale—1 
unipolar instruments recently 
devised by Mr. B. Davies, of 
University College, Liverpool, 
fulfil all the requirements for 
electrolytic work admirably, and 
they have been used for the past 
two years with complete satis- 
faction. The ampere-meter 
possesses two marked properties 
which are especially advanta- 
geous :— 
1. A very open scale, espe- 
cially open for the lower readings. 
2. A practically negligible 
resistance. 
I am indebted to Mr. Davies 
for the following description of 
his instruments :— 
The construction of the volt- 
meter and the ampere-meter de- 
pend upon the same principle, 
the rotation of a coil conveying 
a current around one pole of a 
magnet. The magnetic circuit 
is composed almost entirely of 
iron and steel, with an air-gap 
of 2 mm. in thickness. The 
steel is carefully magnetised and 
‘aged,’ and the circuit is so de- 
signed that the demagnetising 
force is negligible. The chief 
part of the electric circuit is the 
moving coil, which conveys the 
current, or a portion of the 
current to be measured. 
In the voltmeter the coil is 
placed in series with a resistance 
of manganin; in the ampere- 
meter it is placed in parallel with 
a small resistance. The whole 
length of the scale is about 220° 
of arc. Currents of all mag- 
nitudes may be measured from 
ooo ampere upwards, and elec- 
tromotive forces from ;4, volt 
upwards. Both instruments are 
practically ‘dead-beat’ ; this is 
not due to friction, the pivots 
being jewelled, but is an elec- 
tro-magnetic effect. The inter- 
nal resistance of the ampere- 
meters is shown in the following 
table :— 
