HERTZIAN WAVE WIRELESS TELEGRAPHY. 197 



The mercury surface must be covered with water, alcohol, paraffin 

 or creosote oil to prevent oxidation and to extinguish the break spark. 

 The interruption of the primary current obtained by the mercury break 

 is more sudden than that obtained by the platinum contact in air, in 

 consequence of the more rapid extinction of the spark ; hence the sparks 

 obtained from coils fitted with mercury interrupters are generally 

 from twenty to thirty per cent, longer than those obtained from the 

 same coil under the same conditions, with platinum contact inter- 

 rupters. The mercury breaks will not, however, work well unless 

 cleaned at regular intervals by emptying off the oil and rinsing well 

 with clean water, and hence they require rather more attention than 

 platinum interrupters. It is not generally possible to obtain so many 

 interruptions per minute with the simple vibrating mercury inter- 

 rupter as with the ordinary hammer interrupter. The mercury in- 

 terrupter has, however, the advantage that the contact time during 

 which the circuit is kept closed may be made longer than is the case 

 with the hammer break. Also, if fresh water is allowed to flow con- 

 tinuously over the mercury surface, it can be kept clean, and the 

 break will then operate for considerable periods of time without atten- 

 tion. The mercury interrupter may be worked by a separate electro- 

 magnet or by the magnetism of the core of the induction coil. 



The third class of interrupter may be called the motor interrupter, 

 of which a large number have been invented in recent years. In this 

 interrupter some form of a continuously rotating electromotor is em- 

 ployed to make and break a mercury or other liquid contact. In one 

 simple form, the motor shaft carries an eccentric, which simply dips 

 a platinum point into mercury, or else a platinum horseshoe into two 

 mercury surfaces, making in this manner an interruption of the pri- 

 mary circuit at one or two places. As a small motor can easily be run 

 at twelve hundred revolutions per minute, or twenty per second, it is 

 possible to secure easily in this manner a uniform rate of interruption 

 of the primary current, at the rate of about twenty per second. If, how- 

 ever, much higher speeds are employed, then the time of contact be- 

 comes abbreviated, and the ability of the coil to charge a capacity is 

 diminished. 



Professor J. Trowbridge has described an effective form of motor 

 break for large coils, in which the interruption is caused by withdraw- 

 ing a stout platinum wire from a dilute solution of sulphuric acid, and 

 by this means he increased the spark given by a coil provided with 

 hammer break and condenser from fifteen inches to thirty inches, 

 when using the liquid break and no condenser.* 



* See Professor J. Trowbridge, ' On the Induction Coil,' Phil. Mag., April, 

 1902. Vol. III., Series 6, p. 393. 



