TOWNSEND, Circuit Judge

(after stating the facts). Counsel for complainant has critically analyzed the testimony, and contends that the conclusion of the circuit court is erroneous and contrary to the weight of evidence. This contention has been carefully considered. As the court says of some of the testimony: “It is somewhat unsatisfactory, being individual recollections of long-past dates unchecked by any record evidence.” But this is the necessary or natural re suit of a failure on the part of the complainant to assert its rights under this patent until some io years after the occurrence of the events now relied upon to establish validity.

Crot, on whom complainants rely to support the claimed 1885 date of the outside telpher use, contradicts himself as to the date of his employment, at this critical period, to the extent of six months, and asserts in one suit that he had never used carbon brushes before July, 1885, and in the other suit that he had used them between March and May,-1884. On the other hand, five of defendant’s witnesses, and two of complainant’s witnesses called by defendant, fix the date of installation of the telpher plant in 1883 or 1884. It is clear, upon an examination of the whole evidence, that the date of installation was, as found by the court below, prior to February 5, ol88S-

A complicated question is raised by the evidence as to the character of the outside use. Van Depoele conceived the idea that by means of a car suspended on an overhead cable ore might be carried from one mountain top to another, and for the purpose of testing the practicability of this idea he caused such a line ta be constructed from the factory of the Van Depoele Company across a lot, and equipped it with ordinary copper brushes, and with a device for reversing the direction of the motor so that the car might make the trip from one street to another and return to the starting point. When it was *252first started the reversing apparatus bent the brushes so that they would not work. The next day Van Depoele substituted two ordinary electric light carbons, and thereafter the car was run for four or five weeks, whenever Van Depoele wished to show it to customers or to experiment with it. The system remained there from six months to a year and a half. Van Depoele did not part with his control over the system or receive any pay for running it. The court below found as follows: '

“There is no dispute on the proof as to what its use was. As a system for overhead electrical transportation, it was experimental, but as a use of carbon brushes in combination with a segmental commutator on an electric motor it was a substantial public use.”

Counsel for complainant contends that no public use can be a bar to a patent unless there is an element of profit involved or unless the inventor allows the invention to go out of his control. The object of evidence on these points is to distinguish between a use by the public, as in Elizabeth v. Pavement Co., 97 U. S. 126, 24 L. Ed. 1000, for the purpose of experiment, and a use by any part of the public, as in Egbert v. Lippmann, 104 U. S. 333, 26 L. Ed. 755, which is not experimental. The ultimate question is not whether the public used the invention, but whether the use was directed to a development of the invention or test of its practicability. For the purpose of proving the ultimate fact, evidence as to whether the inventor derived a profit, from the use or retained control may be important, but it is not necessarily conclusive. But, where a use known to the public is shown, the proof on the part of a patentee who contends that it is not a bar, “because it was for the purpose of perfecting an incomplete invention by tests and experiments, * * * should be full, unequivocal, and convincing.” Manufacturing Co. v. Sprague, 123 U. S. 249, 264, 8 Sup. Ct. 122, 31 L. Ed. 141.

The telpherage system as a whole was put up as an experiment, not for practical utility or commercial purposes. But there is na evidence whatever that the use of the carbon brushes was experimental. As already stated, Van Depoele caused the carbons to be put in as soon as he found that the copper brushes, failed to work satisfactorily. It does not appear that there was any question as to their operativeness or that they were ever examined or tested.

If there were otherwise any doubt as to whether this was a non-experimental use, it would be resolved in favor of defendant by the testimony of complainant’s witnesses. Van Hoogstrate, one of Van Depoele’s employés, .testifies that Van Depoele understood the adaptability of carbon brushes for electrical motors, where a reversal was desirable in 1883, and describes an experimental use at that tim-e.° Jannus, Van Depoele’s patent solicitor, testifies that in 1883 Van Depoele told him he had ascertained by trial that pieces of carbon would answer very well in such cases-, and that Van Depoele then-understood that mechanical and electrical questions were involved in the success of such brushes. Long prior to this date Van Depoele had successfully used carbon brushes for various purposes. There was therefore no reason for further experiment.

Furthermore, the testimony as to the inside or shop telpher use *253in 1884 is practically uncontradicted. The telpher was a small working model substantially like the outside telpher. It was constructed and operated in 1884, and was publicly exhibited to prospective customers and to large delegations of people. Two witnesses testify that it was equipped with carbon brushes. One witness testifies, “I should think they were copper.” Although this successful public use occurred in the Van Depoele factory, no witnesses were produced by complainant to show that the brushes were not of carbon or that their use was experimental. Inasmuch as the burden of proof on the latter point was. on complainant, and in view of the confessed use of carbon brushes on the outside telpher, this use in the shop must also be considered as proved to be a public one.

This test of the practicability of the proposed ore-carrier system between mountain tops cannot impart the experimental character of its use to a distinct, completed, successfully operating invention, which is a part of said system. Manufacturing Co. v. Sprague, supra.

The other evidence in the case also strongly supports the conclusion of the circuit court. Van Depoele successfully used carbon brushes as early as 1881, and he then said he had used them before. Thereafter he applied for various other patents involving similar devices, but he did not refer to the carbon brush or attempt to patent it till 1887. Meanwhile it had been practically used by him and others in public and private without any claim on the part of any one except Forbes, the English patentee, that such use involved invention. The patent finally issued in 1888, but although carbon brushes were openly and exclusively used by rival manufacturers no right was asserted by Van Depoele during his life nor thereafter by this- complainant until 1895, and even then suit was brought both on the Van Depoele and Forbes patents, and it was alleged that Forbes was the original inventor of the devices patented by him, and which cover all that Van Depoele is now claimed to have invented. It was not until this suit was brought, in 1899, that the priority of Van Depoele over Forbes was asserted. As a part of the evidence in support of that assertion the telpher use was shown. As complainant’s expert, quoted in the opinion of the court below, says: “It was a practical use of the invention, since the motor was used occasionally to propel the car in connection with the experiments on the cable system, which the apparatus was intended to embody.”

In this connection the adverse public uses are material, irrespective of whether such uses occurred more than two years prior to the date of Van Depoele’s application. The' evidence shows that long prior to his application other workers in this field and in various localities had publicly and successfully used carbon brushes for various purposes, but without claiming any patentable novelty in such use. Randall, in his patent of 1880, had described the use of carbon contacts as interchangeable with metal contacts.

Where, of a number of independent inventors working in the same field, one takes the last step which accomplishes the result sought, a strong presumption of invention is raised in his favor. But where a number of workers in a single field, when confronted by an obstacle *254to the development of a device, naturally, and practically contemporaneously, independently substitute one well-known material for another, and, finding that it successfully overcomes such obstacle, use it publicly and privately without any claim of exclusive right, the presumption is raised that such workers rightly regarded the substitution as a mere improvement, a mere choice of material such as would be adopted or selected by the skilled workman, or a double use. Van Depoele’s conduct shows that he too considered that he had not made any invention; that he found the tool ready to his hand, requiring no adaptation to fit it for the new use.

It is unnecessary to consider the confessedly great advantages derived from the use of the carbon brush as a result of the experiments in 1888 and thereafter, or the contention that these are due to the comparatively poor conductivity of carbon, which Van Depoele considered an objection and tried to obviate, or whether the doctrine that an inventor is entitled to all the beneficial uses of his invention should be extended to cover a new, original use discovered by others and unknown to him. These questions relate to occurrences long after the period when Van Depoele, familiar with the use of the carbon brush in place of the copper brush, had abandoned it to the public without reservation or claim. Thereafter the public, through him and other users arid the Forbes patent, had acquired full knowledge of such use, arid had practiced' the same, and intervening rights had arisen.

The sole explanation offered by complainant of Van Depoele’s long delay in asserting his rights is the usual one that he was experimenting and perfecting his invention; and this is not sustained by the evidence. The carbon brush worked as long as it was needed in 1881; that it would work when reversed Van Depoele had ascertained by trial prior to September, 1883. The evidence of investigations to meet the difficulty experienced with uninsulated segments is argumentative and inferential, and rests largely on the statement in the claim of the patent that the commutators should be “formed of separated insulated segments.” But this statement was made in 1887, and the omission of all references to carbon brushes in his patents of 1884 and 1886 are quite as forcibly suggestive of his abandonment of them, especially when coupled with attempts to use the carbons, which had resulted in failure, and the public uses already shown.

That the disclosure of the invention was complete is shown by the statement that a carbon brush would obviate certain objections to the copper brush. Thus, speaking of said 1881 successful use, complainant’s witness Verstraete says: “The brush-holder had a large opening that you could slip in a carbon used for electric purpose and rest on the commutator in the same position as the copper brush was before.”

From a study of the whole case, the conclusion is irresistible that Van Depoele, during the period prior to February 5, 1885, used, but saw no reason to assert any exclusive right to the use of, carbon brushes in the combination afterwards claimed.

The decree is affirmed.