1 68 



SCIENCE-GOSSIP. 



electromotive force produced. This is the principle 

 upon which the induction coil works. 



Fig. 



-Pair of Demonstration Induction Coils. 



Fig. 5 shows diagrammatically the construction 

 and connections of a coil, which will perhaps render 

 the following description clearer. Over a long 

 bundle of soft iron wires T, T, is wound a helix of 

 thick copper wire P, P, double cotton covered, form- 

 ing the primary coil. Over this coil, but thoroughly 

 nsulated from it by a thick ebonite tube, is wound 



up in a number of narrow sections — about three-six- 

 teenths of an inch wide. Each is in a very thin 

 ebonite cell, and these are threaded on the ebonite 

 tube surrounding the primary. One end of each sec- 

 tion is connected to the next, and so on to the outside 

 sections when the final ends are taken out to the 

 dischargers t, t — two insulated metal rods. Between 

 these the secondary discharge takes place. In winding 

 the sections the wire is constantly fed through a bath 

 of melted paraffin wax. 



The electromotive force of the induced secondary 

 current increases, to a certain limit, with the number 

 of turns upon che secondary coil, and as these are 

 enormous it is possible to transform a low potential 

 current flowing through the primary, to a very high 

 potential at the secondary terminals. Now, as we 

 have just seen, an induced electromotive force also 

 depends upon tlie rapidity «ith which the current in 

 the primary circuit is made and broken, and in an in- 

 duction coil this is provided for bymeansof the contact- 



-J^ h L. 



u 



Fig. 6. — Diagram showing Construction of Small Induction Coil and Connections, (i.) 



the secondary"coil S, S, consisting of many miles of 

 thin copper wire, single silk covered. No. 12 gauge 

 wire is generally used for the former, No. 35 or 36 

 for the latter. The battery current enters at Tj, passes 

 across B and H, flows through P, P, and returns to 

 the battery via T„. Immediately this takes place 

 T,T, becomes strongly magnetised, H is attracted to 

 T, and breaks contact at B. T,T, then, if of good 

 soft iron is instantly demagnetised, H flies back and 

 remakes contact — and so on. 



t depends upon the length of spark the coil is to 

 give, how the secondary is wound. Up to two or 

 three inch spark length, it may take the form of one 

 continuous coil from end to end, or perhaps in two 

 sections. There is no gain in having more. The 

 secondary of the coil in fig. 4 is wound in two sec- 

 tions, as is seen by the central partition. As the size 

 of the coil increases and the electromotive force 

 becomes higher, great care must be observed to pre- 

 vent sparking inside ; the secondary is therefore built 



(I.) The writer is indebted to the kindness of 

 Messrs. Macmillan and Co. for the loan of fig. 6. 

 It is taken from Wright's " Induction Coils.'' 



breaker B, H, which has recently taken various 

 improved forms, as will be later described. 



It is an essential point, then, for most X-Ray pur- 

 poses that the time of break should be as short as 

 possible, and anything that will conduce to this end 

 will increase the efiTiciency of the coil. It is for thi.s 

 purpose that a condenser Cj C„, is connected between 

 the ends of the contact-breaker, for since the spark, 

 which is produced upon breaking such an inductive 

 circuit as we are considering, would reduce the 

 electromotive force of the secondary, owing to its 

 prolonging the time of break, the condenser is 

 inserted so that the energy of the spark is used up in 

 charging it. 



The most important points, therefore, to be con- 

 sidered for a good coil are : (I ) an efficient contact- 

 break, (2) thorough insulation of the secondar}' from 

 the primary coil and also of any section from another, 

 (3) a condenser of the right capacity. There is, 

 apparently, no simple law for determining beforehand 

 the best capacity a condenser should have for any one 

 particular coil. Two coils of the same size, wound in 

 the same manner, and with the same size and amount 



