Edison, His Life and Inventions [200]
not only in manufacture, but also in the marketing of the product. These considerations, together with his long-settled principle of striving always to make the best, induced him at the outset to study methods of producing the highest quality of product. Thus he was led to originate innovations in processes, some of which have been preserved as trade secrets; but of the others there are two deserving special notice--namely, the accuracy of mixing and the fineness of grinding.
In cement-making, generally speaking, cement rock and limestone in the rough are mixed together in such relative quantities as may be determined upon in advance by chemical analysis. In many plants this mixture is made by barrow or load units, and may be more or less accurate. Rule-of-thumb methods are never acceptable to Edison, and he devised therefore a system of weighing each part of the mixture, so that it would be correct to a pound, and, even at that, made the device "fool-proof," for as he observed to one of his associates: "The man at the scales might get to thinking of the other fellow's best girl, so fifty or a hundred pounds of rock, more or less, wouldn't make much difference to him." The Edison checking plan embraces two hoppers suspended above two platform scales whose beams are electrically connected with a hopper-closing device by means of needles dipping into mercury cups. The scales are set according to the chemist's weighing orders, and the material is fed into the scales from the hoppers. The instant the beam tips, the connection is broken and the feed stops instantly, thus rendering it impossible to introduce any more material until the charge has been unloaded.
The fine grinding of cement clinker is distinctively Edisonian in both origin and application. As has been already intimated, its author followed a thorough course of reading on the subject long before reaching the actual projection or installation of a plant, and he had found all authorities to agree on one important point--namely, that the value of cement depends upon the fineness to which it is ground.[16] He also ascertained that in the trade the standard of fineness was that 75 per cent. of the whole mass would pass through a 200-mesh screen. Having made some improvements in his grinding and screening apparatus, and believing that in the future engineers, builders, and contractors would eventually require a higher degree of fineness, he determined, in advance of manufacturing, to raise the standard ten points, so that at least 85 per cent. of his product should pass through a 200-mesh screen. This was a bold step to be taken by a new-comer, but his judgment, backed by a full confidence in ability to live up to this standard, has been fully justified in its continued maintenance, despite the early incredulity of older manufacturers as to the possibility of attaining such a high degree of fineness.
[16] For a proper understanding and full appreciation of the importance of fine grinding, it may be explained that Portland cement (as manufactured in the Lehigh Valley) is made from what is commonly spoken of as "cement rock," with the addition of sufficient limestone to give the necessary amount of lime. The rock is broken down and then ground to a fineness of 80 to 90 per cent. through a 200-mesh screen. This ground material passes through kilns and comes out in "clinker." This is ground and that part of this finely ground clinker that will pass a 200- mesh screen is cement; the residue is still clinker. These coarse particles, or clinkers, absorb water very slowly, are practically inert, and have very feeble cementing properties. The residue on a 200-mesh screen is useless.
If Edison measured his happiness, as men often do, by merely commercial or pecuniary rewards of success, it would seem almost redundant to state that he has continued to manifest an intense interest in the cement plant. Ordinarily, his interest as an inventor wanes in proportion to the approach to mere commercialism--in other words, the keenness of his pleasure is in overcoming difficulties
In cement-making, generally speaking, cement rock and limestone in the rough are mixed together in such relative quantities as may be determined upon in advance by chemical analysis. In many plants this mixture is made by barrow or load units, and may be more or less accurate. Rule-of-thumb methods are never acceptable to Edison, and he devised therefore a system of weighing each part of the mixture, so that it would be correct to a pound, and, even at that, made the device "fool-proof," for as he observed to one of his associates: "The man at the scales might get to thinking of the other fellow's best girl, so fifty or a hundred pounds of rock, more or less, wouldn't make much difference to him." The Edison checking plan embraces two hoppers suspended above two platform scales whose beams are electrically connected with a hopper-closing device by means of needles dipping into mercury cups. The scales are set according to the chemist's weighing orders, and the material is fed into the scales from the hoppers. The instant the beam tips, the connection is broken and the feed stops instantly, thus rendering it impossible to introduce any more material until the charge has been unloaded.
The fine grinding of cement clinker is distinctively Edisonian in both origin and application. As has been already intimated, its author followed a thorough course of reading on the subject long before reaching the actual projection or installation of a plant, and he had found all authorities to agree on one important point--namely, that the value of cement depends upon the fineness to which it is ground.[16] He also ascertained that in the trade the standard of fineness was that 75 per cent. of the whole mass would pass through a 200-mesh screen. Having made some improvements in his grinding and screening apparatus, and believing that in the future engineers, builders, and contractors would eventually require a higher degree of fineness, he determined, in advance of manufacturing, to raise the standard ten points, so that at least 85 per cent. of his product should pass through a 200-mesh screen. This was a bold step to be taken by a new-comer, but his judgment, backed by a full confidence in ability to live up to this standard, has been fully justified in its continued maintenance, despite the early incredulity of older manufacturers as to the possibility of attaining such a high degree of fineness.
[16] For a proper understanding and full appreciation of the importance of fine grinding, it may be explained that Portland cement (as manufactured in the Lehigh Valley) is made from what is commonly spoken of as "cement rock," with the addition of sufficient limestone to give the necessary amount of lime. The rock is broken down and then ground to a fineness of 80 to 90 per cent. through a 200-mesh screen. This ground material passes through kilns and comes out in "clinker." This is ground and that part of this finely ground clinker that will pass a 200- mesh screen is cement; the residue is still clinker. These coarse particles, or clinkers, absorb water very slowly, are practically inert, and have very feeble cementing properties. The residue on a 200-mesh screen is useless.
If Edison measured his happiness, as men often do, by merely commercial or pecuniary rewards of success, it would seem almost redundant to state that he has continued to manifest an intense interest in the cement plant. Ordinarily, his interest as an inventor wanes in proportion to the approach to mere commercialism--in other words, the keenness of his pleasure is in overcoming difficulties