Both are in circuit with a local battery B, and the one of greatest resistance, or longest helix, is naturally most effective. Throwing its resistance out of circuit would also intensify the other by permitting more current to flow through the latter. If, therefore, the local battery B1 is brought into action by making a plug connection

at p, its entire current will first pass through the magnet e alone, as f is shunted by the loop from contact spring G, resting against the reed, which offers a path of less resistance, and the current follows 2, 1, b, G, loop, etc. The action of e upon the reed a withdraws it from G, breaking the short circuit and bringing ƒ into play, which, being the stronger magnet, again establishes contact with G, and again cuts itself out of circuit. This alternate action is maintained so long as the local current flows and the reed vibrates in conformity with it. If the vibrating reed a be put in electrical connection with a line wire L, via contact spring I, and a current from battery B be sent along it by depressing key K, the resonator of the same pitch--and only that one--at the far station will emit the same note; which the key K divides into conventional signals. Fig. 9 shows the method of connecting several reeds in telegraphic circuit. A' and B' are the reeds, the vibrating mechanism being omitted to avoid confusion. Each transmitting key, etc., is placed in a loop circuit around its own battery, and the main circuit runs through all, which is a feature of importance in obtaining the intensity of current which the system necessitates.

In conclusion, it only remains to say that the commercial value of such discoveries as the foregoing to telegraph companies, while exceedingly great, is not all that would appear at first glance. The

first inference on hearing that the working capacity of a line had been increased eight-fold would be that the profits were swelled in the same ratio, but this is not the case. To the credit side must be placed, of course, an eight-fold increase of receipts and a diminution to one-eighth of the charge for interest on cost, deterioration and supervision of a system of ordinary type and of equal capacity. Assuming that the average cost, in this country, of a mile

of well-built telegraph line is $250, its mean life-time fifteen years, and that the annual cost for inspection and repair is— which is the case-insignificant; the latter item will not amount to much. On the debtor side must be set down the salary of an additional operator for every time the capacity of the line is increased; since each transmission and reception requires the undivided attention of a clerk. The equipment of a station is also very much increased in cost. There has been, however, no radical increase in the rate of pay of the office personnel since the introduction of multiplex telegraphy, and if the existing scale left a reasonable profit in the days of single transmission, it is clear that stockholders in telegraphs are losing nothing under the new system.

SPIRAL SPRINGS-COMPRESSIBLE AND TENSILE.

BY

OBERLIN SMITH, BRIDGETON, N. J.

SPRINGS, using the word in the mechanical sense only, may be broadly defined as devices to store up power by means of the elasticity of matter. A convenient general classification would divide them into fluid, semi-fluid and solid. The first are, of course, compressible only, and the strain put upon them seems to be merely the crowding closer together of the molecules of a perfectly elastic substance, as air or other gas, in a state of confinement. In what we here call semi-fluid substances, of which india-rubber is perhaps the most notable example, the action appears to be somewhat of the same character, when the material is confined, but to consist chiefly, when not confined, of a very considerable distortion of shape, with a corresponding violent flow of its particles among themselves. In solid springs the particles approach, recede from, or slide past each other, to an amount within a fixed (the elastic) limit, precisely the same as in other solid structural members when subjected to stress. This stress is usually compressive, tensile, lateral or torsional-as regards the spring itself. Relatively to the piece of material of which the spring is composed, the stresses can of course all be resolved, as usual, into compressive, tensile and shearing.

In all the varieties of "flat" and "elliptical" springs the stresses are respectively compressive and tensile on either side of the "neutral line," exactly as in any other beam or lever. Belonging to the same class, but with modified conditions of leverage, are "coiled" (or flat spiral) springs, such as are used in watches, etc., and also such helical spiral springs as are used torsionally, examples of which may be seen in ordinary locks, in mouse-traps and for shutting doors.

It is, however, the object of this paper to treat only of such spiral springs as are strained in the direction of their axes, by either tension or compression. These are usually helical-that is, in the form of a cylindrical coil. The word "helical" would, in fact, have had a more exact meaning than "spiral" in the title of this

paper, as it does not include flat-spirals, but the latter word was used because it is the more popular and better known for the class of springs in question. The popular and expressive terms "pushsprings" and "pull-springs" will herein be used for springs subjected respectively to compressive and tensile stress.

In designing machinery there is frequently a choice between the use of a spiral and some other form of spring. The former is undoubtedly preferable wherever it can be used to as good advantage. It has the merits of cheapness, beauty, compactness, durability and a capacity for uniformity of strain, not usually possessed by other springs. The push-spring is of course the simplest kind, and most desirable to use in cases where either push or pull can be applied to obtain a certain result. Where, in any structure, there happens to be already a hole within which, or a rod upon which, it can slide, it is the cheaper as well as simpler of the two. It has, however, the slight disadvantage of being liable to wear against its inside or outside guiding surface. This the pull-spring is not subject to, and in this respect is undoubtedly superior to all other forms of spring. Of the two most generally used methods of making the hooks at the ends of a spring of this kind, namely, bending the wire itself into a hook or reducing the coils in diameter and closing them around a separate headed hook of larger cross section, the latter is certainly preferable, especially if the spring is subjected to any swinging action which tends to wear the hook.

Regarding methods for coiling spiral springs, I call to mind three in common use. One is to wind the wire into a spiral groove, of the proper section to fit, turned in a mandrel. This is, however, an unnecessary trouble and expense, both in the construction of the tool and in getting the spring off of it. A second and just as satisfactory method is to wind upon a plain cylindrical mandrel, obtaining the requisite pitch of spiral by "feeding" the "tension grip" through which the wire passes along upon the carriage of an engine lathe, or equivalent tool. This is a cheap and convenient plan for machine-shop practice, where comparatively few springs of a kind are required. In cases where large quantities are required there is applicable a third method, which consists of forcing the wire through a spiral die by means of feed rollers. This delivers the springs in indefinitely long pieces.

In regard to material for spring wire (say of one-half inch diameter and below), there is probably nothing, on the whole, so satis

factory as hard-drawn brass. For a cheaper article, hard-drawn iron and steel is good. Of course, more strength and elasticity can be obtained with the same weight of metal by using a tempered steel spring. The latter are not, however, as dependable as the others, on account of being liable to "temper-cracks" and unevenness or too great hardness of temper. They are undoubtedly best in many large springs (as car-springs, etc.), and in all cases where it is desirable to save room and dead weight. The popular prejudice in favor of "steel" springs is, I think, often caused by the prejudiced persons having been troubled with softer springs gradually "setting" under their load in consequence of improper design-there not being enough metal in them to allow of its strains being kept far within its elastic limit.

In respect to the theory governing the action of spiral springs, very little seems to be popularly known. It is a singular fact that few of the best known engineering books say anything about it. Having had occasion for some years past to make numbers of miscellaneous springs, I have endeavored to find some better principle to work upon than the time-honored "cut-and-try." A search through a number of works that happened to be at hand, by Rankine, Clark, Molesworth, Weisbach, Haswell, Nystrom, Trautwine, etc., revealed nothing but a few empirical formulæ regarding safety-valve and car springs-no principles being given. (It is but fair to state, however, that in some of their works Rankine and Weisbach do treat briefly the theory of the matter in question.)

A correspondence with some eminent experimentalists upon strength of materials elicited no information. Communication with various spiral spring manufacturers only developed the fact that this was entirely out of their line, and something in which they seemed to take no interest.

While studying this subject some time ago, the fact was pointed out to me by a friend that the action upon the wire (the word "wire" is herein used in a technical sense, regardless of its size or shape) of a spiral spring, while it is being extended or compressed, is almost purely torsional. Since this paper and some of the experiments below mentioned were commenced, I have seen a small book by John Perry, C.E., lately published by Cassell, Petter & Galpin, in which several pages are devoted to spiral springs, and this torsional action is demonstrated. Credit is given by the author to Prof. Thomson, of Dublin, who, it seems,