408
17.
SMALL CURRENT TECHNIQUE
The ring/e ber electrometers10 of gure 17—23 use a ber
hahging or
stretched between two metal knife edges which are
usually charged to a relatively high xed potential. Motion of
the ber is observed With an ordinary microscope. The capacity
is small (1 to 10 e.s.u.).
The sensitiveness can be made as high
as 300—1000 divisions per volt and a wide range of voltages (0.001
to
100) may be measured.
This instrument, like the Wulf,
Lindemann and Dershem electrometers, has the great advantage
of quick response and absence of oscillations.
The preparation of quartz bers for use in electrometers is
described in experiment 16—1.
D. C. AMPLIFIERS
17—16.
D. C. Ampliers.—The E. P. 54, or “electrometer
tube
”
is a space charge tetrode which has been constructed so as
.
to have an unusually high
{5HŒLD
input resistance.
As dis—
{
cussed in section 6—7, the
! A
!
tube may be used as a d.c.
ê
'
î amplier of any small cur_
5
r;
.
r
;
rent applied across a xed
18
i resistance between its grid
_
£
_
l'
+
_
l'
+
_
+
“L‘ +
:
and cathode.
Very small
_
L---------.ÈY----.‘îY.--.ÊY-‘.….—ÊY
J'
-
:
“
changes in plate current may
_
F1G. 17—24.
'A
amplier.
be observed by
“
_
,
out
”
the
comparatively
large d.c. plate current.
A simple circuit of this type is
'
gure 17—24. The xed resistance R is to be large (say 3000—5000
-.
.
ohms)
in comparison With that of the galvanometer G. Before a
voltage is applied across the input terminals ÂB, the potentiometer
r (say 400 ohms) is adjusted until there is no deection of the gal_
vanome'ter.
It is possible to do so since the plate current of the *
tube‘ows through-G in the opposite direction to that from the -
balancing battery in the lower righ-‘thand corner. . A small current
through n Will now alter the grid potential, which changes the
'
plate
the balance.
The deection of the gal—_
.
yanometèr
Will then be directly proportional to the input current.
_
if thetube is operatedon the straight portion of its plate current—
7
,
:
grid
curve.
'
_