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ENGINEER  AND  INVENTOR  April 1893

The Three-Wire System of Electricity Distribution

Electro-chemistry in 1893
photo  acknowledgement to Pixabay on Pexels

On Saturday last Mr. Justice Romcr delivered judgment in the case of Hopkinson versus the St. James and Pall Mall Electric Lighting Company, Limited. The action was brought by Dr. John Hopkinson for the infringement of his patent, No. 3576 of 1882, as amended in June, 1891, for an invention of "improvements in distributing electricity, and in apparatus to be employed for that purpose."

The use by the defendants of the system was not denied, the defence being mainly confined to the two pleas of anticipation by prior user, and want of invention. The general features of the three-wire system of electric distribution arc so well known that a few words of description will suffice.

Two dynamos are used coupled in series, with the positive terminal of the one connected to the negative terminal of the other. From each of the two outer terminals D and F a main is led into the network, while a third main proceeds from the joined terminals, also into the network. Now, if each dynamo is designed to give a potential of 100 volts, it will be seen that there will be a difference of potential between D and H, and between H and F, of 100 volts, and between D and F of 200 volts. As the mains C, G, and E are coupled to D, H, and F, there will likewise be a difference of potential of 100 volts between C and G, and between G and E, and of 200 volts between C and E.

If the mains are kept at these differences of potential, incandescence lamps requiring a potential of 100 volts can be employed between either set of mains C G or G E. If there be the same number of lamps on each system, there will be no current from the main G to the terminal H, for it is evident that the current that flows along C, and through the two groups K and M, finds its way back to the dynamos through the groups J and L and the main E, and that there is none available to return by the main G. But if one group be extinguished (say L), then the group J will only furnish a passage for half the current from the groups K and M, and the other half must return by the main G. Similarly, if it be the group K that is extinguished, then the necessary current for J must come by the conductor G, which always carries an amount of current corresponding to the difference of load on the two parts of the system. In an extended network in a town there should not be any great difference in the two sides of the system, and hence the current in the third or centre wire should always be small.

The advantages of the three-wire system over the two wire are that the electric current can be delivered at double the potential and with half the current. If the area of conduction be determined according to Lord Kelvin's law, the two outer mains need be only half the weight, while the centre main may be much less than half the weight of each main in a two-wire system. Practically this saving is often exceeded, owing to the size of conductors being increased from that given by the law in order to limit the variation of pressure permissible.

The Edison Company claim that the saving in copper is about 60 per cent. At the date of Dr. Hopkinson' s patent the ordinary parallel two-wire system of electric distribution was, of course, well known, and in addition there was a series parallel system, in which each parallel contained two or more lamps. Here the difference of potential between the mains C and E is sufficient to feed two lamps in series, while the main G serves to equalize the pressure of each group. But if some of the lamps in one group should be turned out, then evidently the remainder would burn more brightly, since they must afford a passage for the current for the entire group opposite. Hence there would be a constant fluctuation, lamps going up and down in brilliancy as adjacent householders altered their lighting. In extreme cases the lamps in one group might be so few, as compared with the lamps in the other group, as to be burned down with excess of current. In that case the entire network would be put into darkness.

Further, it was known that groups of arc lamps in series could be furnished with a return wire which took the difference of current required by the two groups. Messrs. Siemens Brothers had supplied several alternate, current machines constructed to admit of this. All the bobbins in the machine were arranged in series between the X and - terminals at the top. But a connection was also brought out to a terminal at H. When the full number of lamps were burning the conductor G only served to protect the one group from the effect of the fluctuations in the other group, and it was often dispensed with. Its use was to allow the whole of one group to be extinguished without greatly interfering with the other group.

It was one of the aims of the defendants to show that the invention included this arrangement, in which case, of course, the patent would have been invalid. The specification did not refer specifically to paralled distribution, and it was argued that it therefore included series distribution. On the other side it was pointed out that the form of distribution was stated again and again to be one at constant potential, and that neither in 1882, nor now, would any one dream of using an extended arc lighting plant otherwise than in series. We have not the space to follow the evidence on this point, but will quote a part of the summing up to show the effect it had on the mind of the judge;

"Now, it is true that throughout the patentee does not use the term parallel to define the system he is dealing' with. I believe this to be because he preferred to use the appropriate scientific terms to describe the system rather than the popular terms. But, in my opinion, when the specification is raad with care and fairness, and well understood, it is clear that the patentee is thereby confining his claim to the paralled system. He shows throughout that he is dealing only with a system in which the potential is to be kept constant at the places where the electricity is to be supplied, and in which the current varies as more electricity is required to be used on the different wires that tap the main conductor—that is to say, that he is dealing only with the parallel system, for, on the balance of evidence, it is clear that the characteristics I have named denote the parallel system, and only that system.

At the commencement of the specification (page 3 of the print, lines 7 to 10), where he refers in general terms to his invention, he shows that the system he is dealing with is one where the electro-motive force is supplied at a fixed potential. Now, that is a phrase well known to electricians. Moreover, I think that the words such conductors, at line 9, mean conductors for an electric supply at a fixed potential.

It is possible however, that the words may mean merely conductors for an electric supply, and I therefore pass on, only observing that the sentences immediately following (and in particular the phrases on lines 12 to 17 and 24 to 25) appear to me to support the view I had taken as to the meaning of the words such conductors, in line 9. Then at lines 9 and 10 and 31 to 38, the patentee describes what his invention is and its special advantages, substantially as previously pointed out by me in this judgment. At lines 9 and 31 he shows that one of the advantages is economising the cost of the main conductors, and I may add that the very expression main conductors implies branch conductors, which do not really exist except on the parallel system."

After the trial commenced, however, the defendants obtained evidence of the existence in Glasgow, prior to the date of the patent, of a parallel distribution system arranged on the three-wire system. They obtained leave to bring this forward, and put into the box Mr. J. D. F. Andrews, who deposed to using a Siemens alternate-current machine of the kind already referred to, according to the method set forth in a sketch which he had prepared. It will be seen that three mains proceeded from the alternator, and were distributed through the workshop, lamps for lighting being arranged on both sides of the system, as well as lamps that were being run on the pumps, and lamps undergoing the flashing test. This was such a very serious piece of evidence that the plaintiff obtained an extended adjournment for consultation and inquiry. When the court reassembled, Mr. Andrews was severely cross-examined, and admitted that he had applied to Dr. Hopkinson for a license under the very patent under trial. Other evidence was brought to show that the machine was altered after it left the makers to put the halves in parallel instead of in series, and grave doubt was cast on the accuracy of the sketch. On this point the judge said

"Now, if the defendants' case as to Sauchiehall (Glasgow) had not broken down, an anticipation would have been established. But on the whole evidence as to this I am satisfied there was no anticipation, and that the installation at Sauchiehall was not the plaintiff's three-wire system at all, but at most merely a case of the ordinary multiple parallel, where the middle wire is not connected with the dynamos.

There were two witnesses for the defendants (Andrews and Town), whose testimony, if it could have been accepted, would have established that the third wire was connected with the dynamos. Sir Horace Davey in his summing up referred also to a witness, McKillop, but in justice to that witness I am bound to say that, taking his evidence as a whole, he did not purport to say from his own knowledge that the third wire was connected with the dynamos. Now as to Andrews and Town I am sorry to be obliged to say that I cannot give credit to their testimony on the point. This Sauchiehall case broke down, and may be dismissed from further consideration."

The evidence on this point was a conflict of recollections, and naturally the judge gave the plaintiff the benefit of the doubt. To invalidate a most useful and valuable patent on the strength of a half-forgotten arrangement, put up in an unsuccessful experimental workshop in Glasgow more than ten years ago, would have been a serious matter, even if there had been no rebutting evidence; but when the principal witness was flatly contradicted by others, the course of justice was clear. It is probable that Mr. Andrews failed to distinguish between what he had done at a certain date and what he would do now in the light of his present knowledge.

There were two other alleged anticipations—at the Alexandra Palace and at Inchicore. The judge ruled that the Alexandra Palace installation was not a case of the parallel system at all. Two dynamos were coupled; a conductor was led from each arc lamp, and a single conductor served as a common return from both lamps to the joined poles of both dynamos. When both lamps were burning they wer eburnt in series on one main conductor, and the whole electric current went along the main conductor through the two lamps, and for all practical purposes no electricity passed down the return or central wire. When one lamp only was burned, the central wire operated as part of the main conductor, and the electric current went along this one conductor only, through the lamp back to the dynamos. Instead of there being four main conductors to burn two lamps, there were three only. For all substantial purposes the central wire took part of the whole current or none. In regard to this the judge said;

"The practical advantages of the plaintiffs invention were not, or were certainly not all, obtained in this installation. And I think it is not a fair objection to the novelty of a process, which is intended to be and which is of great use when applied to a system comprising many things, to say that the case of one has not been expressly excluded by the inventor, and that as the application of the process to the case of the one, so far as it could be applied to the one was known, therefore the process had been anticipated, though the utility of the invention practically disappears when the case of the one is considered.

If such an objection were allowed there could never be a valid patent for many important discoveries, as for instance, for many discoveries in connection with multiple telegraphy. Then as to the Inchicore case, that in principle is not distinguishable from, and does not go beyond the Alexandra Palace case. The only difference is that instead of one lamp on each side of the central wire there were several lamps. But the lamps were in series—the same number on each side—and, as before, either all the lamps were burnt in series on a main conductor, in which case substantially no current passed down the central wire, – or one set of lani]s was burnt in series, in which case the central wire became part of the main conductor, and substantially all the current flowed down it. In this case, as in the Alexandra Palace case, one of the essentials of the plaintiffs system was absent—namely, the joining of the middle points of the bridges or parallels to one another. All the remarks I have made with regard to the Alexandra Palace case in fact apply to this, and I need not repeat them. It follows that the alleged anticipations fail.

The case of anticipation having been disposed of, there remained the plea of want of invention. It was argued that the arrangement having been carried out at the Alexandra Palace for one lamp on each side of the system, it did not require any skill or inventive power to put several lamps on each side. With all the facts before one there seems a great deal of force in this objection, and yet experience tells us that people of ability will have a ready way out of a difficulty staring them in the face, burning them, as the children say in Hunt-the-thimble, and yet for weeks and months they will fail to recognise it.

All the facts may be known, and yet their significance is not recognised until it is specifically pointed out. The plaintiffs were able to point out that Edison had invented the same system in America immediately after Dr. Hopkinson did so here, and that Professor Silvanus Thompson, one of the defendant's witnesses, had prepared, about the date of the patent, a diagram showing all the principal and newest systems of lighting which were known or occurred to him, and yet it never occurred to him to suggest a system like the plaintiff's. The judge took the common-sense view that it requires thought and experiment to find what many men have looked for unsuccessfully, even if eventually it proves to be full open to the public gaze.

The patent was also attacked on one or two minor points, but we need not do more than note them. It was said that a voltmeter, which was described in the specification, would not work, and that there was insufficient description to enable a workman to employ two alternate-current dynamos with the three-wire system. Both these points were overruled.

Although the action was brought in the name of the inventor, Dr. John Hopkinson, the patent is now the property of the Westinghouse Electric Company, who were the real plaintiffs. At the close of the case they agreed to the suspension of the injunction for six months as a concession and out of regard to the great public inconvenience that would have been caused by a summary stop being put on the electric lighting of the St. James's district. The St. James's Company have, however, to continue the account of profits which the judgment has awarded the plaintiff during the suspensory period.

It will be interesting to know in what position the supply companies stand who use the modified three-wire system. This does not appear to come within the clearly-restricted provisions of Dr. Hopkinson 's second claim, which reads, "Second. In a system of electrical distribution or supply, the employment of three (or more) conductors in combination with two (or more) dynamo machines in series substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawings, whether the middle conductor be continuous or be replaced by an earth return, as set forth." Whether it will be possible to apply the doctrine of mechanical equivalents to embrace the system shown in the sketch is a question for legal determination, but if not, the Westinghouse victory is likely to be shorn of a good deal of its value.

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This article is a reprint of an existing article from Engineer and Inventor, April 1893. It is the intent of this website to present this article in human and machine readable form. Format and editing changes have been made. This article is provided for the purpose of enjoyment only. Statements in this article were relevant to the published period and may not be applicable in current times.