\Forced Vibrations Experimentally Illustrated. 169 



This difference must be attributed to 



(1) the "lag" of the spark, 



(2) imperfect interruption of the primary current, 



(3) the fact that the inductances were determined for 



frequencies considerably below those of the actual 

 magneto circuits. 



If the inductances had been measured for a frequency 

 of several thousands per second the values obtained would 

 presumably have been all considerably smaller, and the 

 theoretical value of the secondary potential would have 

 been correspondingly reduced. 



If the steady-current values of the inductances had been 

 used in the calculation of: the secondary potential the result 

 would, on the other hand, have been considerably greater— 

 probably about 50 percent, greater than the calculated value 

 given above. It is clear, therefore, that only a small pro- 

 portion of the initial magnetic energy JL^'o 2 (Lj being here 

 the primary self-inductance for slowly varying currents) 

 appears as electrostatic energy in the secondary circuit. 

 Eegarded as an arrangement for producing high potential, 

 the magneto is therefore a machine of low efficiency. In 

 view of the fact that high secondary potential is the chief 

 condition for spark production, and that in the opinion 

 of some authorities it is also one of the controlling factors in 

 the process of ignition — it has been suggested that a suffi- 

 ciently high potential will produce ignition even though 

 no spark actually passes, — there appears to be need for 

 improvement in this respect in the design of high-tension 

 magnetos. 



XII. Forced Vibrations Experimentally Illustrated. By 

 E. H. Barton, D.Sc, F.R.S., Professor of Physics, and 

 H. M. Browning, B.Sc, Lecturer in Physics, University 

 College, Nottingham* . 



[Plates VIII. & IX.] 



Contents. Page 



I. Introduction 169 



ir. General Theory 170 



III. Illustrative Experiments . . ." 173 



IV. Detailed Theory 175 



V. Experimental Results 178 



I. Introduction. 



IT is well known that forced vibrations play an important 

 part in most branches of physics. We may mention 

 in this connexion: resonance tubes, fluorescence, Lodge's 



* Communicated by the Authors. 





