cylinder, which is filled with oil or other liquid, is fitted with a piston having a piston-rod passing downwards and terminating in an attachment for the goods to be weighed. As the goods are lifted by the crane the whole of their weight is taken by the liquid in the

Fig. 12 .

cylinder, and the pressure on the liquid, as indicated by a pressure gauge, gives the weight. The gauge has a plain dial, marked off to indications given by the application of standard tons and cwts.; it

could probably be read to about j % of the load weighed. Spring Balances.

For many purposes spring balances are the most convenient of all weighing machines. They arc rapid in action, the indication is in general clear, and there is no need of loose weights except for testing the machine occasionally. Their action depends upon the extension of one or more spiral springs, and as the extension is proportionate to the weight which causes it the graduation is very simple. The accuracy of spring machines depends upon the accuracy of the springs and the workmanship of the machines. The springs in general are very accurate and uniform in their extension, and are very permanent when fairly well used; but their indications are apt to vary from fatigue of the springs if they are kept extended by a weight for a long time. Their indications also vary with the temperature, so that for good work it is advisable that spring balances should be frequently checked with standard weights. For the sake of compactness and convenience of reading the extension of the springs, and consequently the load, is frequently indicated on a dial, by means of a small rack and pinion, which give motion to a finger on the dial-plale, but the regularity and correctness of the indications of the fmger will depend upon the condition of the rackwork and upon the friction, and these will vary with the wear of the machine. For the above reasons spring balances are not in general so accurate as knife-edge machines. It is found that when a spiral spring is extended by a weight it has a tendency to turn a little round its axis. Therefore an index pointer attached to the bottom of the spring, and moving past a scale would rub slightly against the case.

To correct this tendency the spring is usually made half wth right-hand spiral and half with lefthand spiral.

The extension of a spiral spring is given by the formula: —

Extension = W4/!RVE, in which W = weight causing extension, in lbs; n = number of coils; R = radius of spring, from centre of coil to centre of wire, in inches; r = radius of wire of which the spring is made, in inches; E = coefficient of elasticity of wire, in lljs per square inch. The value-of E depends upon the tempering of the wire and will vary accordingly: for the springs of trade balances E will usually be about 10,500,000. For the application of the above formula it is necessary to measure (R) and (r) very accurately, by reason of the high powers involved, but when this has been carefully done the formula may be relied upon. Thus in the case of a spring for which the values of the quantities were W = 7 lb, n = 5i, R =-30m., r = -038 in.,

E = 10,500,000, the formula gives extension = 1 •764 in., while

direct experiment gave extension = 1-75 in. And with a very long

and weak spring for which the values of the quantities were W = J oz., )!=233, R = -35 in., r = -oo85 in., £ = 10,500,000, the formula gives extension =2278 in., while direct experiment with the spring gave 23-5 in.

Atilomalic Weighing Machines.

During the last few years great efforts have been made to expedite the operation of weighing machines by the introduction of machinery, more or less complicated, which renders the machines to a great extent self-acting. The object aimed at varies very much with different machines. Sometimes the object is to weigh out parcels of goods in great numbers of the same definite weight. Sometimes the object is to weigh out parcels of goods, of unknown weight, as in ordinary retail dealing, and to give the exact value of each parcel at different rates per lb.

Sometimes the object is to weigh many loads in succession, the loads being of varying weight, and to present the total weight at the end of a day's v, ork; this is the case with machines for weighing coal and other minerals. Of course the introduction of automatic mechanism introduces friction and other complications, and it is difficult to construct automatic machines that shall be as accurate in their weighing as the simpler weighing machines, but in many weighing operations a moderate degree of accuracy will sufljcc, and speed is of great importance. It is to meet such cases that the greater number of automatic weighing machines have been invented. Some examples of these machines will now be given.

Automatic Computing Spring Weighing Machine for Retail Purposes (fig. 13). — A light and carefully balanced drum with its axis horizontal is enclosed within a cylindrical casing, and rotates freely in bearings formed in the ends of the casing. The casing is fixed in supports on the top of a strong frame, which also carries a small platform machine of ordinary construction on which the goods to be weighed are placed. The pull of the load is transmitted to a hook which hangs freely from the middle of a horizontal bar below the drum casing. At each end of the drum casing is attached a vertical spiral spring, and by the extension of these springs the weighing of the goods is cflcclcd. There are also two vertical racks, one at each end of the casing, in connexion with

the two springs,

and

these actuate pinions on

the axle of the drum and

cause it to revolve as the

springs

extend.

The

horizontal

bar

which

receives the pull of the

load is connected at its

ends with the two spiral

springs and pulla vertically

upon them. Above

the horizontal bar, and

parallel with it, is a rod

which is connected at its

ends with the lower ends

of the vertical racks, and

at its middle with the

horizontal bar. The connexion

with the horizontal

bar is through the

medium of an adjustable

cam.

This cam can be

turned by hand in a

vertical plane by means

of a worm and wheel

movement, and by turning

the worm the vertical

distance between the bar

which is attached to the

springs and the rod which

is attached to the racks

can

be increased

or

diminished, and thus the

racks can be moved relatively

to the springs. By

this means the zero of the

scale on the drum can be

—

^

jt>

->

aHJiistpd to thp fixed

From the Notice issuer! by the Standards Dfpartaa.ustea to tne nxca

^^^^ ^, j, ^^ jj^^, ,^j ^^ ^^^j^^ [^y permcsion oi iht index on the casing when Controller of H.M . Stationery Office. there are no goods on the

platform.

There is also

a compensation arrangement

for effecting automatically the same adjustment for changes of temperature. To deaden the vibration of the springs after a load has been placed on the platform, and thus to enable the weights arid values of the goods to be read rapidly, the piston of a glycerin cylinder is attached to the end of the le'er which pulls upon the hook of the horizontal bar and is worked by it in the glycerin. Fig. 13. — Price-computing Spring

Weighing ^iachine.