after stating th# facts, delivered the opinion:

The object that Dobrowolsky seemed to have chiefly in mind was to avoid the useless converting of energy into heat by the constant and *712rapid changing of poles of the transformer apparatus then in use. As things have turned out, this has not proven the chief actual advantage of the Dobrowolsky apparatus. The actual advantage of the Dobrowolsky transformer — an advantage not denied in an3r quarter — is the. combination in one unitary structure of apparatus to transform the currents, instead of having the transformation go through three separate transformers. Besides this, the amount of iron in the three smaller transformers is ‘ proportionately larger than that required for the Dobrowolsky transformer, so that there is a great advantage in the latter, both with respect to bulk and cost. And besides this,- also, is the saving of the loss of energy.

Now though these advantages be different from the one chiefly in the patentee’s mind, the invention will not on that account fail, if there be in the concept an actual advantage, and the structure embodying it evinces patentable invention; for a patentee is entitled, not only to what he specifically sees, but to what has been brought about by his invention, even though not' at the time actually seen.

But it is said that the transformer of the Dobrowolsky patent is in all essential respects anticipated by the invention of Tesla, and that the claim as written, “closed or nearly closed” includes in principle the transformer of Tesla; that though Tesla’s transformer is not so nearly closed, the difference is a difference of degree only; in short neither transformer being entirely closed, the “nearly closed” of Dobrowolsky and the not closed of Tesla cannot be differentiated.

A reference to the Tesla patent shows that while in the Dobrowolsky patent the magnetic lines of force complete their circuit through the cores of the other coils, in the Tesla patent the magnetic lines of force are completed through the air or open space within the ring. In the Dobrowolsky apparatus the several cores of iron form three or more closed magnetic systems, and the lines of force due to any one of the three coils must complete their circuit through the iron cores of the other two coils, or the lines of force of two adjacent coils must pass in common through a core; in the Tesla apparatus, there are no closed magnetic systems whatever — only a single continuous annular core of iron. The lines of force of any of the coils in the Tesla apparatus cannot by any possibility complete their circuit through the remaining coils. Nor can the lines of force of two adjacent coils complete their circuit through the core.

Now these are differences, not of form, but of function — differences between two magnetic systems, not because one is, in form merely, more nearly closed than the other, but because by reason of the structure, the magnetic lines of force in the one must complete their circuit through the open air, while in the other the circuit is completed through iron. And it is because the conductive of iron differs from the conductivity of open air that the operation of the Dobrowolsky patent is essentially and functionally different from the operation of the Tesla patent.

It is apparent then that the phrase used in the Dobrowolsky claims, “nearly closed,” is not meant as an expression merely of degree. The phrase was meant as the statement of the function of the magnetic sys*713tern. There must, in such unitary system, be enough of the core to bring about unitary action. Beyond that a closed core is not an essential; and to the extent only, that it is not functionally needed, h may, according to the terms of the claim, out of consideration o!, economy or weight, be left more or less open. In short, the phrase, “nearly closed,” was meant solely to convey this thought: That consistent with a system that would complete the magnetic lines of force substantially through iron as distinguished from air, in the manner pointed out, the structure might be left open to save weight and expense ; hut that the limit of the area to be left open is the point where the further presence of iron would be superfluous. “Closed or nearly closed” thus has a distinct functional significance, differentiating it from any apparatus that did not have this purpose in view.

In view of the advantages that have followed transformers of the Dobrowolslcy character, and of its unanticipated method of applying the laws of electricity, (though the laws themselves were well known) we think that the patent in suit evinces patentable invention; and the appellant’s device, coming in every respect within the thought embodied in the patent in suit, the decree ought to be affirmed.