Eee Ee 
OcToBER 2, 1913] 
NATURE 
lated plate, which is kept charged at a constant | 
potential. On varying the inclination of the plate 
by tilting the instrument, a position can be found 
for which the leaf is only just in stable equi- 
librium; slightly increasing the tilt would cause 
the leaf to fly over to the plate. In such a case, 
as is well known, the sensitiveness of an instru- 
ment is very high, a familiar example being the 
suspended magnet galvanometer, in which, by 
adjusting the field magnet, the controlling field is 
so arranged that the magnet is only just in stable 
equilibrium. The capacity of this instrument of 
Wilson’s is very small, and a reading microscope 
attached to the stand enables accurate readings 
to be taken. Fig. 1 shows in section the instru- 
ment as made by the Cambridge Scientific Instru- 
ment Co. 
To avoid difficulties, known to all physicists, 
which occur in working with a gold leaf, Wulf 
Fic. 1.—The tilted gold leaf electrometer. 
has devised a very effective instrument, put on 
the market by the firm of Giinther and Tegetmeyer, 
in which the leaf is replaced by quartz fibres 
rendered conducting by sputtering with a thin film 
of platinum in a kathode-ray tube. Two such fibres 
hang side by side, loaded with a minute weight : 
on being charged the fibres repel one another, and 
the separation is read with a microscope. The 
fibres give a very sharp image, and thus all diff- 
culty connected with reading by one irregular edge 
of a gold leaf is avoided. The capacity is 
smaller than that of the smallest leaf instrument, 
and practically independent of the potential. The 
sensitiveness never approaches that of the tilted 
electrometer, but this instrument is excellent for 
measuring potentials of either a few volts or a 
few hundred volts, according to the fineness of 
the fibres, the size of the weight, and other details 
of construction. It fills the gap between the tilted 
and the Braun electrometer, and is very conveni- 
ent and portable. A somewhat similar design is 
the Einthoven string electrometer, in which a 
silvered quartz fibre is stretched between, and 
parallel to, two metal plates, kept at a constant 
difference of potential. 
NO. 2292, VOL. 92] 
Teo 
Pere) 
The pattern of quadrant electrometer devised 
by Dolazalek is so widely used at present that it 
suffices to mention very briefly the improvements 
introduced, the small dimensions of the needle and 
quadrants, the quartz suspension, the amber 
insulations, and the light needle of silvered paper, 
rendered rigid by its peculiar form. Dolazalek 
has, however, recently devised an instrument 
differing in many important particulars from that 
familiar to English physicists, which he calls the 
binant electrometer, from the fact that the four 
(Q72.) 
Fic. 2,—The binant elecirom ter. 
quadrants are replaced by two semicircular 
“binants ” 1; it is made by Herr Georg Bartels, of 
Gottingen. This instrument has many advantages 
over the quadrant pattern, and is being widely 
used in Germany, although at present it seems to 
be unknown in England. The “needle” is a disc 
formed of two semicircular segments of the thin- 
nest sheet-aluminium, stiffened by means of 
embossed ridges, and insulated from one another 
with amber. The box which encloses them is like- 
wise made up of two semicircular parts supported 
on amber, arranged so that their line of separation 
is perpendicular to 
the line of separation 
of the needle segments. 
Needle and box are 
not plane, but formed 
from shallow concentric 
spherical shells, the 
centre of which coin- 
cides with the point 
of suspension of the 
needle. Owing to this 
simple device an oscil- 
lation of the needle 
does not bring it any nearer to the enclosing walls, 
and the needle is stable at very much higher 
potentials than in the case of the quadrant electro- 
meter; this form also lends increased rigidity to 
the delicate needle. When in use, one of the 
segments of the needle is charged positively, the 
other negatively, by earthing the middle of the 
battery used for charging; contact is made for 
the one segment through the suspension, which is 
an exceedingly fine Wollaston wire, and for the 
other through a still finer coiled wire arranged 
1“ Annalen der Physik,” (iv) 26, 1903. F. Dolazalek, ‘* Binantelektro- 
meter.” Figs. 2, 3, and 4 are from this paper. 
Fic. 3.—The binant electro- 
meter. Plan. 
