This appeal involves the validity of three patents held by plaintiffs for manual and hydraulic cable benders. After a jury trial, the district court held the three patents valid and held that defendant had infringed them by manufacturing and marketing two cable benders, one manually operated and one hydraulically operated. We reverse and remand for further proceedings.

On October 19, 1971 the United States Patent Office issued patent 3,613,430 (the '430 patent) to Silas R. Crees based on an application filed July 25, 1969. This patent was for a hydraulically powered cable bender designed for use within the close confines of electrical transformer boxes. Crees subsequently applied for a patent for a manually operated cable bender. The Patent Office rejected his initial application dated January 17, 1974 because some of the application's claims were vague and some were anticipated by prior patent art.1 Crees amended the application and the Patent Office issued to him patent 3,888,101 (the '101 patent) on June 10, 1975. Before issuing the '101 patent, however, the Patent Office instructed Crees to divide the amended application and seek two separate patents.2 On August 31, 1976 Crees filed for a patent for another hydraulic cable bender based upon the division of the '101 patent. He received patent 4,052,879 (the '879 patent) on October 11, 1977. Crees assigned the three patents to plaintiff Manufacturing Research Corporation (MRC), which manufactured and marketed two different tools, a manual cable bender which was the commercial embodiment of the '101 patent and a hydraulic cable bender embodying the '430 patent.3

Greenlee Tool Company has marketed for several years two devices that are remarkably similar to the MRC manual and hydraulic cable benders. One of Greenlee's largest distributors is defendant Graybar Electric Company, which has marketed Greenlee's manual bender (the 796 Model) and hydraulic bender (the 800 Model) throughout central Florida. In 1977 plaintiffs brought this suit4 against defendant for patent infringement.5 Plaintiffs claimed that the Greenlee 796 cable bender infringed the '101 patent and that the Greenlee 800 cable bender infringed both the '430 and the '879 patents.6 At the conclusion of the evidence, the district court instructed the jurors to presume that the three patents were valid unless Graybar had proven their invalidity by clear and convincing evidence. The jury held for plaintiffs on all counts, the district court denied a motion to set aside the verdict or for a new trial, and judgment was entered.7

Although the factual issues underlying this appeal are exceedingly complicated, most of the controlling legal principles are firmly established. The 1952 Patent Act, 35 U.S.C. §§ 1-293, establishes the conditions of patentability. First, to be patentable an invention or process must be "new and useful." 35 U.S.C. § 101.8 An invention must also be novel and not run afoul of certain specific statutory bars to patentability. 35 U.S.C. § 102.9 The most relevant of these statutory bars for our purposes are the bars which preclude patentability if the invention was either "anticipated" by prior art, or "on sale" or "in public use" more than one year prior to the date of the patent application. 35 U.S.C. § 102(a) & (b). See Hobbs v. United States Atomic Energy Commission, 451 F.2d 849, 855 (5th Cir. 1971). Finally, an invention must not be an obvious development to a person having ordinary skill in the pertinent field of invention. 35 U.S.C. § 103.10

Once a patent application survives the scrutiny of the Patent Office, it is presumed to be valid. 35 U.S.C. § 282;11 Parker v. Motorola, Inc., 524 F.2d 518, 521 (5th Cir. 1975), cert. denied, 425 U.S. 975, 96 S. Ct. 2175, 48 L. Ed. 2d 799 (1976). This presumption of validity "is based on the acknowledged experience and expertise of the United States Patent Office and recognition that patent approval is a species of administrative determination supported by evidence." Ludlow Corp. v. Textile Rubber & Chemical Co., 636 F.2d 1057, 1059 (5th Cir. 1981). Thus, in an infringement suit the burden of establishing patent invalidity as a defense rests with the party challenging the patent. This burden generally is an onerous one. The alleged infringer must demonstrate the patent's invalidity by clear and convincing evidence to succeed in this defense. Kiva Corp. v. Baker Oil Tools, Inc., 412 F.2d 546, 553 (5th Cir.), cert. denied, 396 U.S. 927, 90 S. Ct. 262, 24 L. Ed. 2d 226 (1969); Speed Shore Corp. v. Denda, 605 F.2d 469, 471 (9th Cir. 1979); Astra-Sjuco, A. B. v. United States International Trade Commission, 629 F.2d 682, 688 (C.C.P.A.1980).12 The presumption of validity is severely weakened, however, when pertinent prior art was not considered by the Patent Office in its review of patent applications. Cathodic Protection Service v. American Smelting & Refining Co., 594 F.2d 499, 505 (5th Cir.), cert. denied, 444 U.S. 965, 100 S. Ct. 453, 62 L. Ed. 2d 378 (1979); Gaddis v. Calgon Corp., 506 F.2d 880, 885 (5th Cir. 1975). In those instances, the burden upon the challenging party is lessened, so that he need only introduce a preponderance of the evidence to invalidate a patent. Futorian Manufacturing Corp. v. Dual Manufacturing & Engineering, Inc., 528 F.2d 941, 943 (1st Cir. 1976). See Beckman Instruments, Inc. v. Chemtronics, Inc., 439 F.2d 1369, 1374-75 (5th Cir.) (presumption of validity is seriously weakened, although not entirely dissipated, when pertinent prior art is not before the Patent Office), cert. denied, 400 U.S. 956, 91 S. Ct. 353, 27 L. Ed. 2d 264 (1970).13

Finally, the ultimate determination of patent validity is a conclusion of law. Bird Provision Co. v. Owen's Country Sausage, Inc., 568 F.2d 369, 372 (5th Cir. 1978). This conclusion of law, however, turns on several underlying factual inquiries. Id. As to these subsidiary factual findings in a patent case, our scope of review is restricted by the "general proposition that jury findings on disputed matters of fact will be upheld by the reviewing court if substantial evidence exists to support them." Control Components, Inc. v. Valtek, Inc., 609 F.2d 763, 767 (5th Cir.) (citing Boeing Co. v. Shipman, 411 F.2d 365, 374-75 (5th Cir. 1969) (en banc)), cert. denied, 449 U.S. 1022, 101 S. Ct. 589, 66 L. Ed. 2d 484 (1980).

Graybar argues that the district court erred in refusing to set aside the jury verdict finding the '101 patent valid. Graybar contends, inter alia, that there was no substantial evidence to support the jury's finding that the patented manual cable bender had not been placed on sale more than one year before the patent application. Specifically, Graybar argues that undisputed evidence demonstrated that in October 1972 Henry B. Durham sold to Greenlee the manual cable bender that allegedly infringes the '101 patent, and that this sale bars the subsequent patent application. We agree with Graybar, for there was no substantial evidence in the record to support the jury's finding that no manual cable bender embodying the principles of the '101 patent had ever been placed on sale prior to January 17, 1973, the statutory bar date for the '101 patent. Indeed, uncontroverted evidence establishes beyond doubt that Henry B. Durham had sold such a device to Greenlee in 1972.

Undisputed documentary and oral evidence indicated that Durham met Frank DeBartola, Greenlee Tool's Southern Regional Sales Manager, at a trade show in Birmingham, Alabama sometime in October 1972. The evidence also shows that Durham privately showed DeBartola a model of a manual cable bender that he had built. On November 9, 1972 Durham signed a written agreement with Greenlee submitting his manual cable bender for consideration, and DeBartola sent the tool to Greenlee's product development division for review. The manager of this division soon notified Durham that Greenlee was enthusiastic about the tool, but that Greenlee had to research ways of manufacturing the tool. Greenlee subsequently reversed the cable bending shoes and modified the size and shape of the shoes before entering into an exclusive licensing agreement with Durham on April 16, 1973.

These actions constituted the placement on sale of a cable bender that embodies the concepts of the '101 patent application. The statutory on sale bar applies when the invention that is the subject of a patent application is merely offered for sale; there is no requirement that a sale be consummated before the statutory bar attaches. Arbrook, Inc. v. American Hospital Supply Corp., 645 F.2d 273, 278 (5th Cir. 1981); Application of Theis, 610 F.2d 786, 791 (C.C.P.A.1979). Further, a single sale or offer is enough to bar patentability. In re Yarn Processing Patent Validity Litigation, 498 F.2d 271, 277 (5th Cir.), cert. denied, 419 U.S. 1057, 95 S. Ct. 640, 42 L. Ed. 2d 654 (1974).14 In this case, Durham presented Greenlee with a functional tool that embodied the tool claimed in the subsequent '101 patent application a full fourteen months before the '101 patent application was filed. It is of no consequence that the actual sale was not completed until the licensing agreement was signed in April 1973. The offer for sale alone operates to preclude patentability of the invention claimed in the '101 patent.

Plaintiffs contend that the negotiations between Durham and Greenlee were secret and therefore could not start the statutory time clock running. This contention is without merit, for it is well established that any sale or offer, whether public or private, is enough to implicate the statutory bar. Hobbs v. United States Atomic Energy Commission, 451 F.2d at 860; Piet v. United States, 176 F. Supp. 576, 584 (S.D.Cal.1959), aff'd per curiam, 283 F.2d 693 (9th Cir. 1960). Plaintiffs also contend that Durham sold his idea to Greenlee only for experimental purposes and that the experimental sale exception to the on sale bar applies. We disagree. For this exception to apply the party invoking the exception must prove that the seller intended the product to be used primarily for experimentation rather than for commercial exploitation. Lear Siegler, Inc. v. Ark-Ell Springs, Inc., 569 F.2d 286, 290-91 (5th Cir. 1978); In re Yarn Processing Patent Validity Litigation, 498 F.2d at 286.15 There is virtually no evidence in the record, however, to support plaintiffs' invocation of the experimental sale exception. The November 9 agreement was an unrestricted offer of Durham's tool for review and purchase. The agreement provided that Greenlee had no obligation to retain the tool in confidence, something no inventor still experimenting with his device would be likely to allow. See Ajem Laboratories, Inc. v. C. M. Ladd Co., 424 F.2d 1124, 1126 (6th Cir.), cert. denied, 400 U.S. 830, 91 S. Ct. 59, 27 L. Ed. 2d 60 (1970); Piet v. United States, 176 F. Supp. at 582. Further, the device was complete to the extent that it could be commercially exploited by the purchaser without significant further experimentation. Hall v. Macneale, 107 U.S. (17 Otto) 90, 97, 2 S. Ct. 73, 27 L. Ed. 367 (1883); Application of Dybel, 524 F.2d 1393, 1399-1400 (C.C.P.A.1975). It is of no moment that Greenlee made minor design modifications to facilitate production and to allow for cables of different thicknesses to be bent. A sale is not experimental merely because of subsequent modification. Hobbs v. United States Atomic Energy Commission, 451 F.2d at 859; Mayer v. Mutschler, 248 F. 911, 915 (2d Cir.), cert. denied, 248 U.S. 563, 39 S. Ct. 8, 63 L. Ed. 423 (1918). Where, as here, the invention is complete to such an extent that the purchaser knows how it will operate, it is submitted to the purchaser without reservation, and the party invoking the exception otherwise fails to show that the intended purpose of the sale was primarily experimental, the sale is not experimental and the on sale bar attaches.

Graybar challenges the district court's jury instruction requiring the jury to presume the '430 patent valid unless Graybar proved its invalidity by clear and convincing evidence. Specifically, Graybar contends that it introduced into evidence pertinent prior art that was not before the Patent Office and that this evidence rebutted the presumption of validity. Consequently, Graybar argues, it should only have been required to demonstrate invalidity by a preponderance of the evidence.

As we have stated, the presumption of validity that attaches to an issued patent is seriously eroded when relevant prior art was not before the Patent Office. The question before us is what degree of relevance uncited prior art must have before the presumption is weakened. Plaintiffs contend that the prior art cited by a patent challenger must be more pertinent than the prior art cited by the patent applicant and the Patent Office before the presumption is weakened. They argue further that the challenger's prior art must be outside the patent classification areas cited in the file wrapper16 before the reviewing court may conclude that it was not considered by the patent examiner. See Canaan Products, Inc. v. Edward Don & Co., 388 F.2d 540, 544-45 (7th Cir. 1968). Graybar, on the other hand, argues that there is no requirement to weigh the relative pertinence of the cited and uncited prior art, and that the presumption of validity is weakened if the uncited prior art is at all pertinent to a determination of anticipation or obviousness. Additionally, Graybar argues that it is irrelevant whether the challenger's uncited prior art is within the patent classifications cited in the file wrapper.

We agree generally with Graybar that the presumption of validity is eroded if the patent challenger brings forward any relevant uncited prior art. No case in this circuit or in the former fifth circuit has held that the reviewing court must undertake to balance differing degrees of relevance in assessing prior art and we decline to do so now. Given the extraordinary degree of candor required of a patent applicant, see Beckman Instruments, Inc. v. Chemtronics, Inc., 439 F.2d at 1378-79, and the anticompetetive effects of a patent, see Graham v. John Deere Co., 383 U.S. 1, 5-6, 86 S. Ct. 684, 687, 15 L. Ed. 2d 545 (1966), it is incumbent upon us to apply a strict rule on this issue. Otherwise, a patent applicant would be able to reap the benefits of the presumption of validity without necessarily undergoing the full scrutiny of the Patent Office, when it is that full expert scrutiny that is the reason for the presumption in the first place. Thus, we hold that the "statutory presumption of validity does not apply to prior art not cited to the Patent Office, and even one prior art reference not cited to the examiner overcomes the presumption." Turzillo v. P & Z Mergentime, 532 F.2d 1393, 1399 (D.C. Cir.), cert. denied, 429 U.S. 897, 97 S. Ct. 260, 50 L. Ed. 2d 181 (1976) (footnotes omitted). See also American Seating Co. v. Southeastern Metals Co., 412 F.2d 756, 760 (5th Cir. 1969) (presumption of validity weakened if Patent Office did not have all relevant prior art before it). Our only caveat to this stern rule is that cumulative prior art references by a challenger will not weaken the presumption of validity. Saf-Gard Products, Inc. v. Service Parts, Inc., 532 F.2d 1266, 1271 (9th Cir.), cert. denied, 429 U.S. 896, 97 S. Ct. 258, 50 L. Ed. 2d 179 (1976); Schnadig Corp. v. Gaines Manufacturing Co., 494 F.2d 383, 391 (6th Cir. 1974).

We also reject plaintiffs' contention that a reference in the file wrapper to a patent classification area compels the inference that the patent examiner considered and rejected all prior art in that classification. There simply are too many patents within each patent classification area to assume that a patent examiner will consider carefully every patent within a given classification. As the plaintiffs' own expert witness testified, it is probable that an examiner will sometimes miss highly pertinent prior art in a patent search. Indeed, this case provides such an example. In a patent search on the '101 patent, the examiner failed to turn up Durham's prior patent for an identical manual cable bender despite having searched the relevant patent classification. In light of the potential for error in a patent search, we refuse to attach much significance to a mere reference in the file wrapper to specific patent classifications.17

In this case, Graybar introduced into evidence relevant prior art that was not cited by either the patent applicant or the examiner.18 We have carefully studied these prior art references, as well as those cited in the '430 patent application, and we conclude that they are quite probative of the issues of anticipation and obviousness and are not cumulative. We therefore hold that the district court erred in instructing the jury to apply a clear and convincing evidence standard to the defense of invalidity. Graybar is only obligated to show invalidity by a preponderance of the evidence.

Normally at this point we would apply the correct legal standard to the evidence adduced at trial to determine whether the '430 patent is valid. See Continental Oil Co. v. Cole, 634 F.2d 188, 191 (5th Cir.), cert. denied, --- U.S. ----, 102 S. Ct. 124, 70 L. Ed. 2d 106 (1981). Unfortunately, we are unable to determine whether the '430 patent is obvious in light of prior art. Although the question of obviousness is a legal issue, it turns upon an assessment of certain predicate factual findings. Graham v. John Deere Co., 383 U.S. at 17-18, 86 S.Ct. at 693. The district court here instructed the jury to evaluate the '430 patent in light of the Graham factors and an incorrect burden of proof.19 Consequently, we have neither any explicit detailed fact findings nor any valid findings implicit in the verdict and we are unable to apply the Graham test of obviousness to this patent. Accordingly, we remand the issue of the validity of the '430 patent to the trial court for a new trial.

Graybar also challenges the jury instruction on the '879 patent. Graybar contends that it presented relevant uncited prior art at trial and that the district court erred in instructing the jury that Graybar had to establish the patent's invalidity by clear and convincing evidence.

We agree with appellant that the district court incorrectly instructed the jury. As we have already stated, the presumption of patent validity is seriously undermined when a patent challenger produces relevant and noncumulative prior art that was not cited to or by the patent examiner. In this case, the '879 patent application failed to cite two extant patents, as well as a hydraulic thinwall bending device that had been on the market for several years. This prior art was clearly pertinent to a determination whether the '879 patent should issue and was not cumulative.20 One patent in particular, the Simonsen 2,382,266 patent, was especially pertinent, for it appears to operate on a similar principle as the '879 patent. The Simonsen patent involves the application of direct hydraulic pressure upon a cable bending knuckle to bend cable that was parallel to the entire holding apparatus prior to the beginning of the bending process. Because the patent application failed to disclose this prior art, we hold that the presumption of validity was weakened and the district court should have instructed the jury to determine the defense of invalidity by a preponderance of the evidence standard.

Again, we cannot determine on appeal whether the '879 patent was an obvious development to a person having ordinary skill in developing construction tools. There were substantial conflicts in the testimony at trial concerning the scope of the prior art as well as the differences among the prior art references and the claims forming the basis of the '879 patent. In light of the erroneous jury instruction and the lack of detailed factual findings that are necessary for a determination of the obviousness or lack of obviousness of the patent, we reverse the judgment of the district court and remand for further proceedings.Conclusion

We have concluded that the '101 patent is invalid because a substantially identical tool was on sale more than one year prior to the date of the '101 patent application. Consequently, the judgment of infringement must fall as well.

We have also concluded that the district court incorrectly instructed the jury on the burden of proof needed to invalidate the '430 and '879 patents. Accordingly, we reverse the judgment of the district court and remand for a new trial as to the two patents. In view of this disposition, we do not reach the issue of infringement. The question of obviousness is intimately bound up with the infringement issue, and we think it inadvisable to attempt to sever the questions artificially at this time. See Dazenko v. James Hunter Machine Co., 393 F.2d 287, 291 & n.7 (7th Cir. 1968) (limited remand appropriate only when issues are so separate as to ensure no injustice would result).21

REVERSED AND REMANDED.