1028

THE COLLIERY GUARDIAN.

November 19, 1915.

public welfare. There was one rather striking point, as
shown by the curves exhibited by Prof. Bone. While
the curve showing consumption evidenced a constantly
progressive increase, the export curve showed a very
much more rapid proportional increase. That rather
pointed to the fact that we were still able to hold our
own with the more favourably placed countries as
regards coal supplies. This question of economy was
one which they, as colliery people, might be just as
keen in supporting as their friends in the ceramic
industry. The colliery proprietors were supposed to
have the whip hand, but he could assure them that it
was not an easy task in these days of heavy legislative
burdens, high wages, and increasing natural difficulties
to produce coal at a reasonable price. When they con-
sidered that in that district the bulk of the coal was
being worked half a-mile below the surface, and that
the more remunerative seams were being fast exhausted,
it necessarily followed that the coal would maintain
a high market value, and for that reason it was more
than ever necessary that the user, whether he be the
colliery proprietor, the iron master, or the earthenware
manufacturer, must exercise the greatest possible
economy in the use of it.

Mr. W. Simons said the price of coal was increasing'
very materially, and after the war the price would not
go back relatively to anything like what people thought.
Therefore, coal being the bread of industry, people would
be compelled to economise more in the future than in
the past. There was an enormous waste going on in
some of the iron and steel works, and coal was being
used in a heathenish manner. After the war steps
would have to be taken to remedy that. In addition to
the passing of the resolution which they had adopted, he
suggested that the committee which Prof. Bone referred
to should form a committee of experts, to advise the
various industries on this subject. In defence of the
British iron and steel industries, it should be stated that
the adoption of by-product ovens had been delayed
because of the prejudice in favour of beehive working,
which was supposed to produce better iron. He believed
that that was only prejudice, which was rapidly being
overcome. Figures showed that greater progress was
being made now that that prejudice was being worn
down. He proposed a vote of thanks to Prof. Bone, and
this was seconded by Mr. T. W. D. Gregory, and
carried unanimously.

Prof. Bone briefly replied.

THE INSTITUTION OF ELECTRICAL
ENGINEERS.

MANCHESTER SECTION.

CHAIRMAN’S ADDRESS.

At the meeting of the Manchester section of the
Institution of Electrical Engineers, on Tuesday last,
the Chairman (Mr. B. ’Welbourn) delivered an address
on “ The Production and Properties of Electrolytic
Copper.” After treating on the sources and production,
the consumption and the smelting of copper, the address
proceeded to deal with

Rolling and Wire Drawing.

For the transmission of electricity, only two varieties
of copper are required :—(1) Hard-drawn copper, which
comprises all conductors in which mechanical strength
is required, such as (a) trolley wires for tramways; (b)
catenary and contact wires for railways; (c) aerials for
wireless telegraphy; (d) wires for overhead transmission
lines. (2) Soft or annealed copper, which comprises all
conductors which are to be continuously insulated, and
in which the highest obtainable conductivity is required.
Bus-bars for switchboard work may either be hard-
drawn or annealed. In reducing the wire to the
required diameter, the number of dies or “ holes ”
through which it is drawn depends on whether the wire
is to be used “hard ” or “ soft,” and, if the former,
on what tensile strength and other mechanical pro-
perties are needed. Generally speaking, the greater
the number of holes or dies the higher is the resultant
tensile strength.

When hard-drawn wire is needed, it is then ready for
use either as a single wire, e.g., a trolley wire, or it can
be stranded into a more flexible conductor for use on an
overhead transmission line. If soft copper is needed,
the hard-drawn copper is usually annealed in a cast
iron retort, which is heated externally, and which is
sealed by water at both ends to prevent the entry of
air through the descending mouthpieces. The retort
is filled with water vapour under pressure at a tempera-
ture of about 800 degs. Fahr, for small wires, and
1,100 degs. Fahr, for larger wires; and the copper which
is to be annealed is carried slowly through the heated
tube by means of an endless conveyor.

Jointing Wires.

Where long lengths of wire are required, it is
frequently necessary to joint wires together during
manufacture. In the smaller sizes which are used in
the construction of insulated cables this may be done
conveniently and economically by the use of electrical
butt-welders, and these joints will stand further drawing
through dies. For trolley wires, some engineers prefer
that copper wire bars of suitable weight should be used
to ensure getting the length required without any joints.
An alternative to this is to joint the rolled rods (from
which the wire is to be drawn) by means of scarfed
joints which are brazed together with silver as the
solder, and extensive experience of this method shows
that it is entirely successful. Both .this and the
electrically welded joint referred to above will stand
drawing down through the dies, and, in the finished
wire, the joint cannot usually be detected by the eye,
nor is its tensile strength any less than that of the rest
of the wire; in fact, it is usually higher.

Testing and Definition of Hard-Drawn Copper.

The testing of annealed copper conductors is very
simple, and is confined to gauging the wires with a
micrometer gauge, and measuring the ohmic resistance,
and sometimes to the weighing of samples or coils. In
the case of hard-drawn copper, additional tests are
necessary to determine :—

1.	Tensile strength.

2.	Extension as a test of hardness.

3.	Limit of, proportionality (elastic limit).

4.	Absence of brittleness.

Tests (1), (2), and (3) are usually made simultaneously
on a specimen which is exactly 10 in. long between the
grips of a Dennison, Buckton, Avery, or other suitable
testing machine. Modern testing machines are
arranged so that autographic records of the extension of
the wire may be taken which are sufficiently accurate
for commercial work. If, however, exact extension
measurements are needed, then they must be made
carefully by an experienced worker with an extenso-
meter. In practice, it is found that the actual tensile
strength increases proportionately to the diameter, and
not to the sectional area, as one would expect, although
the tensile strength in tons per square inch increases
with a decrease of diameter. After analysing a number
of tests made by Mr. Thomas Bolton, Mr. A. P. Trotter
found that the tensile strength could be expressed as
follows :—

T = a - b D.

Mr. D. R. Pye and other investigators have found
from a large number of tests that the value of a is
approximately 30, and that of b is 20 on all sizes of
conductors up to 0-5 in. diameter.

It is not, however, sufficient to define the strength of
the wire. It is also necessary to have some check on
its hardness and absence of brittleness by ascertaining
that it does not fall below a stated minimum extension
per cent. Tests on wires from 0-075 to 0-5 in. diameter
show that the minimum extension can be expressed by

E = 5 D.

On these results Mr. Pye has suggested (Journal of
the Institute of Metals, No. 2, 1911, vol. vi.) the
following as the basis of a satisfactory definition of
hard-drawn copper :—“ Hard-drawn copper, when in
the form of circular wires, shall have a tensile strength
not less than that given by the formula :—■

T = 30 — D

and an extension per cent, on a marked 10 in. length,
including point of fracture, of not less than that given
by the formula :—

e = 5 x D

where, in the foregoing formulae :—

T = tensile strength in tons per square inch of
original section.

e — extension per cent.

D — diameter of the circular rod in inches.

An examination of the standard figures prepared by
the Wire Committee of the American Society for Testing
Materials (1909) for wires ranging from 0-04 to 0-46 in.
diameter shows that they agree closely with the above
specification for strength, while the minimum extension
figures agree closely with the formula :—

e — 4 ^D.

The Engineering Standards Committee might, there-
fore, consider the adoption of Mr. Pye’s specification
on the grounds that :—

(a) It calls for a high standard which can easily be
obtained by the leading manufacturers throughout the
world.

(5) Tables of the minimum tensile strength and
extension per cent, can be prepared for use for all the
usual sizes of wires; and

(c) It is easily applied to all intermediate sizes.

Test (4) is almost wholly covered by test (2), and
probably could be displaced by it.

The British Post Office and other users specify as a
ductility test for wires which range in diameter from
0-0791 to 0-2237 in., that the wire shall be capable of
being wrapped in six turns round wire of its own
diameter, unwrapped, and again wrapped in six turns
round wire of its own diameter in the same direction
as the first wrapping, without breaking. A twist test is
also specified, the wire to stand 20 twists in 6 in.; but
in practice, the work done on the wire in twisting seems
to increase its hardness and brittleness, until it becomes
useless.

The General Post Office tests for brittleness give a
rough measure of the quality of the wires, and their
value lies in the ease with which they can be applied,
but the Department does not specify any extension test.
It may be stated, however, that wire which complies
with Mr. Pye‘s specification will also automatically
comply with the Post Office specification. Apart from
this, it does not seem clear why a wire which is to be
erected and subjected to longitudinal strain only should
be tested by wrapping and twisting, and the extension
test would seem to be a more scientific way of deter-
mining the toughness of the metal. This opinion is
confirmed by the action of the Wire Committee of the
American Society for Testing Materials in offering expla-
nations of their omission of these wrap and twist tests
from their specification for hard-drawn copper wire.

Elasticity of Copper.

In view of the extensive and increasing use of copper
conductors on overhead lines, railways, and tramways,
and the great importance of the elastic properties of the
wire in relieving strains, it seems desirable that any
definition of hard-drawn wires should also include a
reference to its “ limits of proportionality ” (elastic
limit) as it does in many foreign specifications, and also,
at first sight, to its modulus of elasticity. There are,
however, practical difficulties in making these delicate

measurements in works routine, and further reference
will be made to these points.

The ratio of elastic limit/breaking strength, i.e., the
limit of proportionality, varies from about 50 per cent,
for wires of 0’08 in. diameter, to about 70 per cent, for
wires of 0-5 in. diameter, when measured on 10 in.
samples. The Engineering Standards Committee might
stipulate for a minimum limit of proportionality for all
hard-drawn wires, and state the conditions under which
the test should be carried out. These should include :
(1) A statement as to the temperature, although the
percentage extension does not appear to be affected by
variations within the atmospheric limits in the United
Kingdom; (2) the type of apparatus and the conditions
under which it should be used; (3) a statement of the
length of the test piece.

An excellent basis for the above is to be found in the
Engineering Standards Committee’s Report No. 55,
which contains the results of an investigation carried out
by the National Physical Laboratory on some of the
standard sizes of wires usdd by the British Post Office.

The Institution of Electrical Engineers (through the
Research Committee) would confer a great boon on the
electrical industry if it would carry through an investi-
gation on similar lines for all standard circular wires
from No. 5 S.W.G. (0-212 in. diameter) to 7/0 S.W.G.
(0-5000 in. diameter). The values could then be inter-
polated for all other sizes of wire by using a large scale
curve which embodied the results of the two investiga-
tions.

The investigation should include work on stranded
conductors, since there seems -reason to believe that the
limit of proportionality is higher for a given section of
stranded conductor than it is for the corresponding
section of solid conductor.

On all strands containing more than three wires, a
considerable number of tests show that the effective
strength should be taken as 10 per cent, below that of
the same number of straight wires. This appears to be
due to the difficulty of getting the strain evenly dis-
tributed among the wires of a strand by any form of
grip, and to the fact that the layers of wires are of
unequal length, especially in the case of a large strand.

The subject of the elasticity of hard-drawn conductors
has not attracted much general attention in this country,
but important references to it have been made, notably
by Mr. W. B. Woodhouse and Mr. G. Carr, the latter
of whom has urged that 200 lb. wires should be made
the minimum allowable on main telegraph lines, because
“ the limit of elasticity is seldom reached ”; and it is
noteworthy that the Board of Trade electrical adviser
has since debarred the use of any smaller conductor
than this on an overhead high pressure line.

In the United Kingdom the regulations of the Board
of Trade provide that the conductor shall be erected
with such a minimum sag that at 22 degs. Fahr., and
with a wind pressure of 25 lb. per square foot of effective
area, the stress in it (excluding its elasticity) shall not
be more than one-fifth of the breaking load, and that
an accumulation of snow may be ignored. The rule
has the virtue of being simple and on the safe side for
English conditions, but it results in abnormal and
unnecessary dips on small wires, of which No. 11|

S.W.G. is the smallest allowed.

It may be suggested for consideration that the proper
procedure would be to decide what would be the worst
conditions under which a conductor will have to work,
and then to erect it, so that under these conditions it
will not be stressed beyond its limit of proportionality.
For instance, the engineers of some of the most impor-
tant Canadian power transmission lines have decided,
after long experience, that for the ordinary sizes of
conductors used, the worst conditions may be taken as
j in. thick of sleet collected on the wire at 32 degs. Fahr,
and a wind pressure of 11 lb. per square foot of effective
area, and that under these conditions a factor of safety
of two is required, and this is within the limit of pro-
portionality.

But, whether the sag under the worst conditions is
calculated one way or another, the fact remains that the
elasticity of the wire ought to be taken into consideration
in arriving at the correct sag to be given when the wire
is erected, so that, under the worst conditions, the sag
will not be greater than that calculated. The wire
cannot possibly be erected under the assumed worst
conditions, and it has to be sagged on comparatively still
days at temperatures of, say, 50 degs. Fahr, to 70 degs.
Fahr.

Suppose a wire has been erected with the requisite
sag under the worst conditions, then when the wind
drops the wire is relieved of a certain amount of stress,
and it contracts by an amount depending upon the
stress it has been relieved of, and the modulus of elasti-
city, but in the very act of contracting it diminishes the
sag, and this again puts a greater stress on the wire,
and tends to stretch it back to its original position;
consequently, the actual sag taken up by the wire is a
matter of somewhat intricate calculation.

Again, suppose a wire is hanging with a certain sag
at a certain temperature, and the temperature rises, say,
20 degs. Fahr., the length of wire in the span will
increase by an amount depending on the coefficient of
expansion of the metal. This lengthening would
increase the sag, but increasing the sag reduces the
tension in the wire, and again the wire, by reason of
its elasticity, will contract by a certain amount depend-
ing on the stress it has been relieved of by the lengthen-
ing of the wire and the modulus of elasticity.

It is rather a long and wearisome matter to calculate
from the worst sag under windage and low temperature
what the erecting sag will be at, say, 60 degs. Fahr.,
with no wind; but the following formulae will be found
both useful and accurate, and they afford a ready means
of obtaining the result. It may be said that the most
convenient way to use the second formula is by assuming
values of d2, and plotting a curve from the values of
so obtained, from which curve can be read the sag at
any required temperature.