With the advent of the automobile and its far greater capacity of burden the platform scales formerly used to weigh loads drawn by horses have become obsolete, and with the re building of such weighing machines automatic recording devices have been added. co that a modern scales of this class will weigh a load of coal approximating five tons within the limit of error of three ounces, and print the correct weight upon a tape without the services of any attendant.
Self-discharging gxales are also much in use for measuring units of weight; as for instance the filling of packages of commodities, as coffee, suger, flour, cereals, etc. The same idea is em bodied in receiving scales in large establish ments, where coal, feed-water, sand, cement, ore, crushed stone, etc., are weighed as de livered, the receiving hopper discharging when the weight enuals one ton, 100 pounds, or any other desired amount.
Railroad or track-scales are of prodigious size and capacity. The largest of this type in the world is a 'suspended-platform' scales in the \Vest Albany yard of the New York Cen tral Railroad. The weighing rail is 90 feet long, and the capacity is 225 tons.
A new idea in heavy-work scale construction is known as the flexure-plate scale. In this type thin plate fulcrums of chrome-vanadium steel take the place of the usual knife-edge pivots. They operate by bending instead of frictional movement on bearings. They have been found of most dependable accuracy and unequaled endurance, being unaffected by dampness or changes of temperature, and thus entirely free from the variations common to the ordinary track-scales.
The conveyor scale weighs material as it passes on a conveyor belt making use of elec tricity instead of weights for the recording. The belt is so arranged in reference to the weighing apparatus that the full belt is weighed against the empty return belt. A vertical plun ger in a tube partly filled with mercury rises and falls in the tube according to the weight of material passing. The moving mercury brings more contact points into action the greater the thrust of the plunger, and the varying cur rent makes its record automatically on a travel iifg tape. A standard comparison of amperes per hour and tons per hour gives the figure by which the weight is readily computed.
The computing scale is a machine which carries an attachment for indicating a certain limited range of prices. Usually it carries a cylinder covered with a paper roll ruled in many different proportions, each ruling indic ative of a certain price unit. The cylinder turns with the weight, thus showing the cost at several different prices per pound.
The difficulty of exactitude in weighing is recognized and a legal 'tolerance,' as it is called, is permitted, within which no one can be accused of dishonesty or fraud. These tol erances are as follows: On counterpoise scales, one grain on one-fourth ounce; two grains on one ounce: 10 grains on 10 ounces: 10 grains on one pound; 40 grains on 10 pounds; 100 grains on 50 pounds. On platform scales one ounce is allowed on 50 pounds: one pound on 800 pounds; two pounds on one ton. On railroad of track-scales the permissible error is four pounds per ton.