2 04 POPULAR SCIENCE MONTHLY. 



the secondary terminals of the coil, and to cut them up into long and 

 short periods in accordance with the letters of the Morse alphabet. 

 This is done by means of the primary key. The primary key generally 

 consists of an ordinary massive single contact key with heavy platinum 

 contacts. As the current to be interrupted amounts to about ten 

 amperes and is flowing in a highly inductive circuit, the spark at break 

 is considerable. If the attempt is made to extinguish this spark by 

 making the contacts move rapidly away from one another through a 

 long distance, in other words, by using a key with a wide movement, 

 then the speed at which the signals can be sent is greatly diminished. 

 The speed of sending greatly depends upon the time taken to move 

 the key up and down between sending two dots, and hence a short 

 range key sends quicker than a long range key. If it is desired to 

 use a short range key, then some method must be employed to extin- 

 guish the spark at the contacts. This is done in one of three ways : 

 Either by using a high resistance coil to short-circuit these contacts, or 

 by a condenser, or by a magnetic blow-out, as in the case of an electric 

 tram-car circuit controller. Of these, the magnetic blow-out is probably 

 the best. 



Mr. Marconi has designed a signaling key which performs the 

 function not only of interrupting the primary circuit, but at the same 

 time breaks connection between the receiving appliance and the aerial. 



The author has designed for signaling purposes a multiple contact 

 key which interrupts the circuit simultaneously in ten or twelve differ- 

 ent places. The particular point about this break is the means which 

 are taken to make the twelve interruptions absolutely simultaneous. 

 If these interruptions are not simultaneous, the spark always takes 

 place at the contact which is broken first, but if the circuit is inter- 

 rupted in a dozen places quite simultaneously, then the spark is cut up 

 into a dozen different portions, and the spark at each contact is very 

 much diminished. By this break, voltages up to two thousand volts 

 may be quite easily dealt with. 



Various forms of break have been devised in which the circuit is 

 broken under oil or insulating fluids, but, generally speaking, these 

 devices are not very portable, and a dry contact between platinum sur- 

 faces with appropriate means for cutting up the spark and blowing it 

 out so that the mechanical movement of the switch may be small is the 

 best thing to use. 



The signaling key is really a very important part of the trans- 

 mitting arrangement, because whatever may be the improvements in 

 receiving instruments, it is not possible to receive faster than we can 

 send. A great many statements have appeared in the daily papers 

 as to the possibility of receiving hundreds of words a minute by 



