NATURE 
| Vov. 9, 18382 
The Umdhlebi Tree of Zululand 
THE word ‘‘umdhlebi” does not, I think, appear in Déhne’s 
** Zulu-Kaffir Dictionary.” I presume it to be a derivative from 
the root 4/asa, which Dohne interprets as denoting, among other 
things, the giving of pain. Some native tales of the tree will 
be found in part iv. of Bishop Callaway’s ‘‘ Religious System 
of the Amazulu,” in which it is asserted that ‘‘ there are several 
kinds, not one kind only of umhlebe; some are small.” I 
should be disposed to think the kernel of fact will be found to 
lie in native observation of the deleterious properties and weird 
aspects of certain Zzphorbiacee. H. M. C. 
Charlton, November 4 
The Weather 
THE past month has probably been one of the wettest on 
record. I have registered here 5°14 inches of rain during the 
month ; only on seven days out of the thirty-one has the gauge 
shown less than 0°1; and on three days out of the seven rain 
has been recorded, J. M. Fountain 
Hillingdon, Uxbridge, November 2 
ON THE GRADUATION OF GALVANOMETERS 
FOR THE MEASUREMENT OF CURRENTS 
AND POTENTIALS IN ABSOLUTE MEASURE 
HERE are several methods by which galvanometers 
may be graduated so as to measure currents and 
potentials in absolute measure, but they all involve, 
directly or indirectly, a comparison of the indications of 
the instrument to be graduated with those of a standard 
instrument, of which the constants are fully known for 
the place at which the comparison is made. There are 
various forms of such standard instruments, as, for example, 
the tangent galvanometer which J oule made, consisting of 
a single coil of large radius, and a small needle hung at its 
centre, or the Helmholtz modification of the same instru- 
ment with two large equal coils placed side by side at a 
distance apart equal to the radius of either; or some form 
of “dynamometer,” or instrument in which the needle of the 
galvanometer is replaced by a movable coil, in which the 
whole or a known portion of the current in the fixed coil 
flows. The measurement consists essentially in deter- 
mining the couple which must be exerted by the earth’s 
magnetic force on the needle or suspended coil, in order 
to equilibrate that exerted by the current. But the former 
depends on the value, usually denoted by 4, of the hori- 
zontal component of the earth’s magnetic force, and it is 
necessary therefore, except when some such method as 
that of Kohlrausch, described below, is used, to know the 
value of that quantity in absolute units. 
The value of may be determined in various ways, 
and I shall here content myself with describing one 
or two of the most convenient in practice. The easiest 
method is by finding (1) the angle through which the 
needle of a magnetometer is deflected by a magnet placed 
in a given position at a given distance, (2) the period of 
vibration of the magnet when suspended horizontally in 
the earth’s field, so as to be free to turn round a vertical 
axis. The first operation gives an equation involving the 
ratio of the magnetic moment of the magnet to the hori- 
zontal component // of the terrestrial magnetic force, the 
second an equation involvinz the product of the same 
two quantities. I shall describe this method somewhat 
in detail. 
A very convenient form of magnetometer is that devised 
by Mr. J. T. Bottomley, and made by hanging within 
a closed chamber, by a silk fibre from 6 to 10 cms. 
long, one of the little mirrors with attached magnets 
used in Thomson’s reflecting galvanometers. The fibre 
is carefully attached to the back of the mirror, so that 
the magnets hang horizontally and the front of the 
mirror is vertical. The closed chamber for the fibre 
and mirror is very readily made by cutting a narrow 
groove to within a short distance of each end, along a 
. fire. 
piece of mahogany about 10 cms. long. This groove is 
widened at one end toa circular space a little greater in 
diameter than the diameter of the mirror. The piece of 
wood is then fixed with that end down in a horizontal base- 
piece of wood furnished with three levelling screws. The 
groove is thus placed vertical; and the fibre carrying the 
mirror is suspended within it by passing the free end of 
the fibre through a small hole at the upper end of the 
groove, adjusting the length so that the mirror hangs 
within the circular space at the bottom, and fixing the 
fibre at the top with wax. When this has been done, the 
chamber is closed by covering the face of the piece of 
wood with a strip of glass, which may be either kept in 
its place by cement, or by proper fastenings which hold it 
tightly against the wood. By making the distance be- 
tween the back and front of the circular space small, and 
its diameter very little greater than that of the mirror, 
the instrument can be made very nearly “dead beat,” 
that is to say, the needle when deflected through any 
angle comes to rest at once, almost without oscillation 
about its position of equilibrium. A magnetometer can 
be thus constructed at a trifling cost, and it is much more 
accurate and convenient than the magnetometers fur- 
nished with long magnets frequently used for the deter- 
mination of 47; and as the poles of the needle may always 
in practice be taken at the centre of the mirror, the calcu- 
lations of results are much simplified. 
The instrument is set up with its glass front in the 
magnetic meridian, and levelled so that the mirror hangs 
freely inside its chamber. The foot of one of the levelling 
screws should rest ina small conical hollow cut in the 
table or platform, of another in a V-groove the axis of 
which is in line with the hollow, and the third on the 
plane surface of the table or platform. When thus set up 
the instrument is perfectly steady, and if disturbed can in 
an instant be replaced in exactly the same position. A 
beam of light passing through a slit, in which a thin ver- 
tical cross-wire is fixed, from a lamp placed in front of 
the magnetometer is reflected, as in Thomson's reflecting 
galvanometer, from the mirror to a scale attached to a 
lamp-stand, and facing the mirror. The lamp and scale 
are moved nearer to or farther from the mirror, until the 
position at which the image of the cross-wire of the slit 
is most distinct is obtained. It is convenient to make 
the horizontal distance of the mirror from the scale for 
this position if possible one metre. The lamp-stand 
should also have three levelling screws, for which the 
arrangement of conical hollow V groove and plane 
should be adopted. The scale should be straight, and 
placed with its length in the magnetic north and south 
line, and the lamp should be so placed that the incident 
and reflected rays of light are in an east and west vertical 
plane, and that the spot of light falis near the middle 
of the scale. To avoid errors due to variations of 
length in the scale, it should be glued to the wooden 
backing which carries it, not simply fastened with drawing 
pins as is often the case. 
The magnetometer having been thus set up, four or 
five magnets, each about 1ocms. long and ‘1 cm. thick, 
and tempered glass-hard, are made from steel wire. This 
is best done as follows. From ten to twenty pieces of 
steel wire, each perfectly straight and having its ends 
carefully filed so that they are at right angles to its length, 
are prepared. These are tied tightly into a bundle with 
a binding of iron wire and heated to redness in a bright 
The bundle is then quickly removed from the fire, 
and plunged with its length vertical into cold water. The 
wires are thus tempered glass-hard without being seriously - 
warped. They are then magnetised to saturation in a 
helix by a strong current of electricity. A horizontal 
magnetic east and west line passing through the mirror is 
now laid down on a convenient platform (made of wood 
put together without iron and extending on both sides 
of the magnetometer) by drawing a line through that — 
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