Basil Graves

It has occurred to me that not a few of our readers would appreciate a brief contribution upon this subject in general, if I may be permitted to judge from the number of queries dealing with the matter which have been reiterated during the past twelve months. The induction coil is one of those modern appliances now of more than mere theoretical interest, so numerous are its applications, not to mention the numberless instances where its use is necessitated. Not a few instances conducive to its universal use are afforded in the method of igniting the explosive charge of internal combustion engines, its utility in radiography, and, to a certain extent, electro-therapeutic work. Nor must we omit to consider the value of a coil as far as work in experimental research is concerned. It constituted the key to the innumerable experiments conducted by Hertz in his endeavors to solve the problem of the Berlin Academy of Science - viz., to establish experimentally any relation between electromagnetic forces and the dielectric polarization of insulators. The investigations which he pursued with a view to effecting this purpose resulted in the discoveries which, I need scarcely say, laid the fundamental basis of that modern application of the effects of electric oscillations - namely, wireless telegraphy. Here again the induction coil becomes of paramount importance; it constitutes the very heat of the apparatus employed. In spite, however, of these facts, the induction coil has continued to remain, comparatively speaking, an expensive piece of apparatus. Perhaps, from the manufacturer's point of view, I am not justified in making such a statement, for it certainly is to be confessed that the requisite time and trouble entailed in the construction of coils - especially with regard to those of the larger type - is considerable. But there are many who, given the means wherewith to proceed, and the necessary time, prefer themselves to attempt the work of construction. It is to thesepersons individually that the purchased article appears so expensive, and to whom, in particular, these remarks apply. Small coils are comparatively simple to make, such as those capable of giving a 1/2 in. spark for the ignition work of internal-combustion engines, or 1-in. as suitable for working in conjunction with ozonizers or illuminating tubes for spectrum analysis. Coils capable of giving sparks between 5 and 12 in. in length are in demand for work in radiography and X-rays, while those yielding sufficient potential difference between their secondary terminals to spark across an intervening air-gap still greater than this last, are suitable for theurapeutic work with high-frequency apparatus, and signalling without wires over long distances.

With regard to the smallest coils, these may well be wound in sections containing continuous layers of wire; but with the larger type of coil it is necessary to subdivide these sections. Taking the former type into consideration first, those up to the 1/4 in. size may be wound in two sections - i. e., the secondary is divided into two equal portions by a central intervening disc of insulating substance; 1 in. coils, again, should be wound in four sections, and 2 and 3 in. in six. Above this size the method of winding must be different and will be discussed hereafter.

The secondary wire utilized for the purpose should in all cases be single silk covered; cotton, besides possessing comparatively poor insulating properties, requires much more space than silk. It may here be said, however, that a double covering of silk is superfluous, no benefit being derived from its Use. Prepar-tory to commencement, let the bobbins of wire be well warmed to effectually cause the evaporation of any moisture contained in the insulation. They should then, if possible, be kept in a warm, dry place until the moment when required for use.

The paper utilized for this insulation of the respective layers of the secondary should be thin and thoroughly dry. It should not, however, be soaked in wax previous to use; it is far preferable to cover the layers with unwaxed paper to commence with, and so soak the whole winding in wax for 24 hours after completion. Thereason of this is that it is difficult to closely cover a layer with paper that has been previously soaked in wax, and, moreover, when this is done, the paper occupies about three times more space than in the unwaxed state. The wire may, however be fed through a small trough of wax, with the result that the insulation will take up a sufficiency of this latter to saturate the paper when, after completion, the whole secondary is heated in a paraffine bath.

The layers of wire should never be wound so close to the edge of a section that there is any risk of one turn slipping below the level of its neighboring turns, for in the event of this occurring the high potential difference which would ensue between the adjacent layers would inevitably result in internal sparking and consequent failure. It is best to discontinue the winding of all layers at a distance of about 3-32 in. from the extremities. This will obviate any risk of such occurrence. The secondary must not, of course, be wound directly on to the ebonite tube, as this latter would not stand the heat if soaked in wax. The best mode of procedure under the circumstances is to make a thin tube of shellacked paper to exactly fit over the ebonite tube, and by means of paraffined discs of card-board, to divide it into the requisite number of sections. This tube is then mounted on the winder and is ready to receive the wire; then, after completion, the whole secondary is ready to be slipped bodily over the ebonite tube containing the primary and core. With reference to the direction of winding of the respective sections, each section should be wound so that the current will be in the same direction through all sections.

Before giving definite particulars as to the specific dimensions of the various parts of small coils, let me make one more remark applicable in general to these - namely, as to the winding of the primary. Where possible this operation should be performed with the aid of a lathe, or other suitable winding appliance, in order that it may be done as evenly as possible; and to effect economy as regards space, let both layers be wound with right-handed helices, as in the case of the primary of larger coils, in order that the successive turns of the second layer may occupy the grooves formed by those of the first. These points as to the primary winding are important, in order that the latter may be a good fit within the insulating tube without unnecessary waste of space.