196 POPULAR SCIENCE MONTHLY. 



We have first the well known hammer interrupter which continental 

 writers generally attribute to Neef or Wagner.* In this interrupter, 

 the magnetization of the iron core of the coil is caused to attract a soft- 

 iron block fixed at the top of a brass spring, and by so doing to inter- 

 rupt the primary circuit between two platinum contacts. Mr. Apps, of 

 London, added an arrangement for pressing back the spring against 

 the back contact, and the form of hammer that is now generally em- 

 ployed is therefore called an Apps break. 



As the ten-inch coil takes a primary current of ten amperes at six- 

 teen volts when in operation, it requires very substantial platinum con- 

 tacts to withstand the interruption of this current continuously without 

 damage. The small platinum contacts that are generally put on these 

 coils by instrument makers are very soon worn out in practical wireless 

 telegraph work. If a hammer break is used at all, it is essential to 

 make the contacts of very stout pieces of platinum, and from time to 

 time, as they get burnt away or roughened, they must be smoothed up 

 with a fine file. It does not require much skill to keep the hammer 

 contacts in good order, and prevent them from sticking together and 

 becoming damaged by the break spark. 



By regulating the pressure of the spring against the back contact, 

 by means of an adjusting screw, the rate at which the break vibrates 

 can be regulated, but as a rule it is not possible, with a hammer 

 break, to obtain more than about 800 interruptions per minute, or say 

 twelve a second. The hammer break is usually operated by the mag- 

 netism of the iron core of the coil, but for some reasons it is better 

 to separate the break from the coil altogether, and to work it by an 

 independent electromagnet, which, however, may be excited by a cur- 

 rent from the same battery supplying the induction coil. For coils 

 up to the ten-inch size the hammer break can be used when very 

 rapid interruptions are not required. It is not in general practicable 

 to work coils larger than the ten-inch size with a platinum contact 

 hammer break, as such a butt contact becomes overheated and sticks 

 if more than ten amperes is passed through it. In the case of larger 

 coils, we have to employ some form of interrupter in which mercury or 

 a conducting liquid forms one of the contact surfaces. 



The next class of interrupter is the vibrating or hand-worked mer- 

 cury l)reak, in which a platinum or steel pin is made to vibrate in and 

 out of mercury. This movement may be effected by the attraction of 

 an iron armature by an electromagnet, or by the varying magnetism of 

 the core of the coil, or it may be effected more slowly by hand. 



* Du Moncel states that MacGauley of Dublin independently invented the 

 form of hammer break as now used. 



See ' The Alternate Current Transformer,' Vol. II., Chap. I., J. A. Fleming. 



