generated in a High-tension Magneto. 157" 



in parallel with the gap, the gap being in this experiment 

 2*31 mm. wide between zinc electrodes. In this curve 

 the first arch (counting from the right) has been much 

 reduced in importance — a feature characteristic of increasing 

 coupling, — its intersection with the second (at Ci = 6*2 micro- 

 farads) being, however, well marked. The curves of figs. 3 

 and 4, when compared, illustrate the fact that if the oscillating 

 current in the secondary coil changes from one of non-uniform 

 to one of uniform distribution (as it does when the secondary 

 terminals are connected with a condenser) the coupling is 

 increased *. In these two experiments the primary coil was 

 in the same position within the secondary, and the currents 

 employed were not very different. In the experiment of 

 fig. 4 the coupling was 0'815. 



A number of other such curves have been determined for 

 an induction-coil; and in all cases in which the comparison 

 has been made substantial agreement has been found, in 

 regard to the form of the curves and the values of u at which 

 the various maxima and minima occur, between the experi- 

 mental curves and those calculated from the function U sin <£, 

 although it should be remembered that in the calculated curves 

 no account is taken of the effects of the damping resistances. 



Consequently the form of the curve, when determined, 

 enables us to estimate the coupling ; and if the primary self- 

 inductance is also known, we can calculate the oscillation 

 constant of the secondary coil. 



4. Determination of the " Capacity-Potential " Curve for 

 a Magneto. 



With these objects in view I have attempted to determine 

 the capacity-potential curve for a certain H.T. magneto. 

 The machine was of the rotating armature type, with the 

 condenser attached to the armature and rotating with it. 



The armature having been fixed so that with the timing- 

 lever in its most advanced position — a position slightly 

 in advance of that shown in fig. 2 — the contacts were just 

 separated, preliminary measurements by Rayleigh's method 

 gave the following values for the self-inductances at about 

 the same ampere-turns in both circuits : — 



Lx = 0*0153 henry at 1/50 ampere, 

 L 2 = 30-7 „ „ 1/2500 „ 



The mutual inductance L 2 i, by comparison with a standard, 

 was found by a ballistic galvanometer method to be 0'64 henry 



* Compare Phil. Mag. April 1014, Table II. p. 570. 



