Low Frequency and their Resonance. 327 



f, g. One of its ends is rigidly attached to the wooden frame 

 of the apparatus, the other end is attached to a lever h which, 

 worked by a micrometer screw, varies the tension of the vibra- 

 tor. There are three mercury cups, a, o, c, and three dippers 

 (which unfortunately do not appear in fig. 1). The middle cup, 

 c, is fixed in position, and the middle dipper, being at the 

 nodal point of the vibrator, makes a permanent contact there. 

 The other two dippers make contact with mercury cups which 

 can be raised or lowered by means of a nut and screw as rep- 

 resented in fig. 1, and indicated in diagram 2. The construc- 

 tion of the adjustable mercury cups and the stretching lever 

 were copied from Dr. Max Wien's magnetic interruptor 

 (Wiedem, Ann. 1891 and 1892). The middle cup (see fig. 

 i), is connected to one pole F, of the gravity or storage cell, 

 the other two cups are connected one to one end, and the other 

 to the other end of the primary of the small coil A, B. From 

 the middle point, C, of the primary a wire leads to the other 

 pole of the cell. Auxiliary small coils, E and D, and conden- 

 sers, H and Gr, are inserted in the circuits as indicated. Their 

 functions will be explained further below. 



The vibrator vibrates with a node at the middle dipper as 

 soon as the tension has reached a certain, by no means high, 

 limit. A permanent contact is therefore maintained at this 

 point, and the contact is made at one of the other cups just at 

 about the same moment as it is broken at the other cup. 

 Leaving the condensers out of consideration for the present, it 

 is evident that this form of the current make-and-brake pro- 

 duces the same effect upon the iron core of the coil as an 

 alternating current would. The advantage of this needs no 

 comment ; for although the iron core consists of the finest 

 iron wire that can be obtained in the market, yet it must be 

 remembered that the vibrator is expected to work sometimes 

 at the rate of 512, or more, complete periods per second. 

 Another immediate advantage which this interruptor offers is 

 a considerable diminution of sparking. The addition of con- 

 densers, besides performing other functions which will be 

 discussed presently, reduces the break sparks almost to invisi- 

 bility, even when currents as large as half of an ampere are 

 used. Each half of the primary coil consists of 532 turns of 

 ISTo. 22 silk-covered wire wound over an iron core of 30 cm in 

 length, 4 acm in cross section, and consisting of very fine, soft 

 iron wire. 



The vibrator when at work gives a pure, but not objection- 

 ably loud, musical note in which the overtones are scarcely 

 perceptible. The frequency ordinarily employed in my work 

 is 256 complete periods per second, and it is obtained by 

 bringing the vibrator in unison with a Konig standard tuning 



