The expansion and improvement of the art of reinforcing concrete began several decades ago. There were two different needs, one was for strengthening concrete beams, and the other for strengthening concrete columns. The term 'reinforcing bar' is usually applied in the art to a rod or bar used to reinforce a concrete beam against the tension longitudinal of the beam itself. The concrete itself has great power of resisting compression, but it has not tensile strength, and its weakness manifests itself in cracking along the lower half of the beam. A steel bar placed below the middle of the beam furnishes the useful tensile resistance. But it was found that certain diagonal cracks developed in the beam toward the respective ends of the beam, and it was sought to prevent these by minor auxiliary rods attached to the main reinforcing bar. This was the field which Vandenburgh entered and devised an arrangement, shown in Figure 1 of his drawings, of a reinforcing bar imbedded in the lower part of the beam with transverse loops or spirals, part on one side of the center of the bar and part on the other and fitted rigidly into the bar so that those on each side leaned at an angle away from the center and in an opposite inclination from those on the other. In strong construction, these spiral loops were doubled by a second spiral which meshed with the first. Now, as the specifications show, this arrangement of the bar and opposing spirals was to apply the truss principle to the strengthening of the beam, and to give it the tensional resistance along the lower part of the beam and against the diagonal tensions near the ends of the beam. No one can read the specifications and examine the drawings without perceiving that this was the gist of the Vandenburgh invention. Reference is made to the use of the plan for column construction, but the only use of it for that purpose is shown in Figure 6, which discloses two upright reinforcing bars with their spirals meshing, but connected in no other way.

The truss formation was not especially adapted to the needs and strains of the concrete column. The history of that field in the art shows that one of the pioneers was a Frenchman named Considere, who some years before the plaintiff's patent had shown that the tendency of concrete in a column was to expand outwardly under the vertical pressure to which it was chiefly exposed and the best way of reinforcing columns was by pouring the liquid concrete into a hooping made of a series of independent hoops, or into a continuous helicoidal spiral or cylindrical helix of a steel wire or rod, of a stovepipe form. After him, column hooping became common in the art as the only practical method of column reinforcing and the convolutions of the spiral were spaced and supported by steel uprights with which they engaged in various ways. In one French patent the convolutions were tied with steel wire to the spacers. It is the form of this engagement between the uprights and the spirals which is the crux of the present suit.

The preferred form of Vandenburgh for the engagement between his reinforcing bar and the spiral loops of his truss arrangement is by a kerf or cut in the edge of bar, inclined away from its center, in which the coil rod is placed, and held therein by an integral spur forced downward upon the coil when inserted, which permanently retains it in position. The defendant has a rectangular cut in the edge of the T-spacer or upright and retains the spiral rod in it by peening or hammering down the edges of the cut, so as to keep the rod from slipping out, but leaving play enough to permit the collapsing of the spiral and shipment in its collapsed form. Others had adopted a similar form of engagement. Observing this, Vandenburgh, who had not used or exploited his original patent in any way, went back to the Patent Office and secured a reissue in which he was permitted to broaden his first and second claims with respect to the kerfs and spurs, so as to change the word 'rigidly,' used in the original claim, to 'permanently.' Having done this, he proceeded to sue in the Second, Third, and Sixth circuits persons using what he insisted were equivalents of his form of engagement in their column hooping reinforcement of concrete columns. All the courts, and there are six of them, have held these changed claims of the reissued patent to be void, and to give him no right to claim infringement in the collapsible feature of the column hooping. The original specifications leave no doubt that the patentee used the word 'rigidly' deliberately and properly, because his truss, as he portrayed it, required rigid connection between the spiral loops and the main reinforcing bar. His making his form of engagement loose was an afterthought, to catch makers who had not been advised in his specifications that there was anything collapsible in his truss formation. The reissue as to the first two claims was in the teeth of the admonitions of this court, speaking though Mr. Justice Bradley in Miller v. Brass Co., 104 U. S. 350, 26 L. Ed. 783. They are therefore void.

Counsel for the patentee say now that the collapsible feature was only an incidental matter, and that what they now rely on is the combination under claims 3 and 5 of the reinforcing bar provided upon one edge with a series of kerfs, each having an integral overlapping spur and a coil convolution thereof being beneath one of the spurs. We do not think the respondent has this combination. We think the patentee's combination must be limited to a spiral with one bar in the truss formation, or at least one in which the spiral is free at one end and has no second support or spacer such as respondent uses. Patentee relies on Fig. 4 of his drawing, and says that to add another spacer or bar is only the work of a mechanic; but he cannot properly say so, because Fig. 4 is only a detail of Fig. 1 of the drawings, and was intended to be a lighter construction of the same beam, with the same truss combination as shown in Fig. 1. Moreover, it is perfectly obvious, from the description and some of the claims, that the spiral of Vandenburgh was intended to be free of all but the bar shown. This is made certain by Fig. 6, which is the only application of the device to column use shown, and in that we have two bars standing upright, each engaged with convolutions of a spiral; the loops of each spiral meshing with those of the other spiral, but free of the other bar. It is clear to us that Vandenburgh, having secured a patent for a truss form of reinforcement and finding it unworkable, for it never has been adopted in the trade or in any structure, is through reissue seeking to expand the paper combination he claims into a field in which it does not belong.

But it is insisted that Vandenburgh was the first to introduce into the field of concrete reinforcing the kerf and integral spur to clamp the spiral rod, that this involved invention, and that claim No. 3 should be construed to secure him a reward for this. It may be true that in the field of reinforcing concrete the kerf and spur had not been used before as Vandenburgh used it, but the kerf and spur were old in the art in kindred fields. They were old in metal-working art. Exactly the equivalent is shown in sand screens for mixing the materials of concrete and in sustaining fence wires. It is difficult to differentiate the field of metal working from this art of reinforcing concrete, because the problem was only one of spacing firmly the convolutions of the metal spiral, and that was a well-known device for such a need. We do not think the principle of Potts v. Creager, 155 U. S. 597, 15 Sup. Ct. 194, 39 L. Ed. 275, applies in this case. More than this, in the very field itself, there was a prior German patent of Kesserling & Moller, showing the use of the kerf for spacing a spiral in reinforcing concrete. It does not seem to us that it involved real invention merely to add a spur or clamp, or to peen or hammer down the edges of the kerf, so as to fix the spiral rod firmly. We find, therefore, that claims 3 and 5 were without merit as involving invention, and that the action of the Circuit Court of Appeals should be

Affirmed.