November 12, 1009] 



SCIENCE 



689 



in a revolving mirror, the musical arc shows 

 serrations which are strongly marked near 

 that pole which would be positive for the un- 

 interrupted arc. Therefore to minimize the 

 effects of convection as well as those due to 

 wandering of the discharge over the terminals, 

 and consequent change of pitch of the note, 

 the poles should be about 8 mm. in diameter, 

 vertical, and the positive one below. 



With the above apparatus, the are, after 

 burning until the positive pole is sufficiently 

 coned, may be made to give a clear note of 

 tolerably uniform pitch and audible through- 

 out a fairly large room. 



Both pitch and intensity depend not only 

 upon capacity, self-induction and resistance 

 in the secondary, but also upon the potential 

 difference of the arc terminals. High poten- 

 tials give clearer tones, but the 110-v. circuit 

 answers very well. For a given secondary, a 

 slight adjustment of arc-length or of resist- 

 ance in the arc-circuit may make a striking 

 difference in the clearness and intensity of the 

 note. Using the 110 V.D.C. mains, a current 

 of 1.5 to 2.5 amperes is necessary. Too much 

 current produces a hissing or an impure note, 

 or even none at all. The fact that the tones 

 may not be pure' does not interfere seriously 

 with their use qualitatively as indicators of 

 changes made in the various circuits em- 

 ployed. All connections should be very firmly 

 made. 



To show induced currents, incandescent 

 bulbs may be used to advantage. Those of 

 small resistance and for small potential differ- 

 ence are better, though of course easily 

 burned out. Add to the above apparatus a 

 few coils of various sizes, some metal plates, 

 etc., and interesting demonstrations become 

 at once possible. They depend on the change 

 of pitch of the arc-note, or on the lighting up 

 of incandescent lamps; these effects arising 

 from modifications of the electrical conditions 

 of secondary or tertiary circuits. 



An obvious experiment is to vary the note 

 by changes in the secondary, several octaves 

 being easily obtained. This makes possible 

 rough comparisons of self-inductions and of 



'Austin, loc. cit. 



capacities for oscillatory currents, by com- 

 paring the arc-tones with those from tuning 

 forks. The use of the ordinary formula, 

 2Tr\/LC for the period of the discharge in- 

 volves the assumption of its applicability, as 

 well as the further one that the resistance 

 may be neglected. It may readily be demon- 

 strated that the latter is only approximately 

 true. 



The short-circuiting of the coil in the sec- 

 ondary produces a note the shrillness of 

 which gives an instructive idea of the part 

 played by inertia in an oscillating electrical 

 system. An incandescent lamp may be made 

 to light up by joining it with a coil laid on the 

 one in the secondary circuit. Rotating and 

 sliding the upper coil are modifications that 

 suggest themselves at once. For oscillations 

 of very high frequency, the bulb will not light 

 up, since the heating effect varies inversely as 

 the frequency, if the latter is high and the re- 

 sistance is negligible. Placing a coil in open 

 circuit on the one in the secondary produces 

 no effect; but the result of closing the circuit 

 of the upper coil is to raise the pitch of the 

 note, the increased frequency of the oscilla- 

 tions arising from the decrease in effective 

 seK-induction in the oscillating system. 



It is instructive to close the upper coil al- 

 ternately through each of two equal resist- 

 ances, one of which is non-inductive. The 

 difference of pitch may be made very striking. 



If two coils are used in series in the second- 

 ary, the effective loading of the oscillating 

 circuit depends on the position of one coil 

 relative to that of the other. The maximum 

 inductance, and consequently the lowest 

 note, is obtained when one coil lies upon the 

 other so that the directions of their fields are 

 the same. This contrasts sharply with the 

 high note emitted when the fields oppose. 



An iron plate laid on the coil in the second- 

 ary increases the inertia of the system and 

 consequently lowers the pitch of the tone. In 

 this way it may be shown that a soft iron 

 plate changes the arc-tone more than a steel 

 plate of equal thickness does. A plate of non- 

 magnetic metal raises the pitch. Such a 

 plate may be regarded as a series of closed 



