562 POPULAR SCIENCE MONTHLY. 



nection^, which, Professor Braim claims, practically avoids all atmos- 

 pheric disturbances.* The details of the receiving arrangement are as 

 follows : The coherer tube consists of an ebonite tube containing hard 

 steel particles of a uniform size, placed in the adjustable space between 

 two polished steel electrodes. It is found that with this steel coherer, 

 a small amount of magnetism in the particles increases its sensitive- 

 ness, and to obtain this, a ring magnet is employed in connection with 

 a coherer tube. Eeceiving apparatus arranged on this system is said to 

 have been used for telegraphing between Heligoland and Cuxhaven, 

 a distance of thirty-six miles. 



All the immense experience, however, gained by Mr. Marconi and 

 those who have worked with his system, is in favor of using the earth 

 connection. There is no doubt that Hertzian wave telegraphy can be 

 conducted over short distances by means of totally insulated aerials, 

 but for long distances the earth connection is essential, for the reasons 

 that have been explained previously. 



There are many of the details of the receiving arrangements which 

 remain to be considered. If the communication is received by a tele- 

 graphic instrument like the Morse printer, which requires a current 

 of anything like ten milliamperes to work it, then an important ele- 

 ment in the receiving arrangement is the relay. The relay that is 

 generally used is a modified form of the Siemens polarized relay, 

 which is so adjusted as to make a single contact. For marine work on 

 board ship, it is essential that this relay shall be balanced so that varia- 

 tions in position shall not afEect it. Sometimes the relay is hung, in 

 gimbals like a compass, and at other times suspended from a support 

 by elastic bands, so as to avoid jolting. In any case, the relay must be 

 so adjusted that no change of position will cause it to close the circuit 

 of the telegraphic printer or recorder. Its sensibility ought to be such 

 that it is actuated by a tenth of a milliampere, and, if possible, even 

 by less. The alteration of sensibility in the ordinary contact form of 

 relay is the pressure that is necessary to bring the platinum points of 

 the circuit closer together, so as to pass the minimum current which 

 will work the telegraph printer. 



The important matter, however, in connection with the use of the 

 relay in Hertzian wave telegraphy, is that it should be capable of adjust- 

 ment without extraordinary skill. It is no use to put into the hands 

 of an operator a relay which requires abnormal dexterity to make it 

 work at all. 



* There is a good deal of contradiction between various inventors on this 

 point, some saying that 'earthed ' aerials obviate atmospheric electrical dis- 

 turbances, and others that insulated aerials are in this respect superior. The 

 truth appears to be that neither form is absolutely free from risk of dis- 

 turbance by this cause. 



