639 



of the receiver purposely. This is not performed by admitting air 

 around the string. On the contrary tlie vacnniii is kept as high as 

 possible in all experiments, but the strength of the magnetic field is 

 changed. By varying this value from zero to a maximnni the amount 

 of the decrement can be adjusted in a simple and at the same time 

 very precise manner. 



In the above fig. 3 the photos are reproduced of the same string 

 as that of fig. 2 the strip of paper moving with the same velocity, 

 i. e. 10,75 millim. per sec. Continuous waves the length of which 

 was 7,5 kilom. were coupled inductively with the circuit of the 

 string, and switched on and off repeatedly, whilst in the successive 

 photos tiie current exciting the field-magnet was increased from 0,5 

 to 6 Amp. The photos show, that liie lime in which the vibrations 

 of the siring die away is shorter as the exciting current increases. 

 The decrements can only l)e measured in the photos A, B and C 

 because of (he low speed of the paper strip. For JJ and E they 

 have been calcidated from the intensity of the magnetic field, which 

 amounted tot 7600 and 10.900 Gauss respectively. 



In the calculations of all useful decrements in radio-telegraphy, 

 the proper decrement of the string itself caused by its internal 

 friction is to be neglected, whilst in a high vacuum also the air 

 damping is not to be taken into account. Under these circumstances 

 tiie relation between tlie decrement and the field intensity is given 

 by the formula 



4 ƒ/' X 10-9 

 <f=~. r^ (1) 



where d represents the logarithmic decrement, 



H the intensity of the magnetic field in Gauss, 

 tn the mass of the string in grams per centim., 

 w The resistance of the galvanometer circuit in Ohms per 



centim., 

 N the number of periods per sec. of the string wiien vibrat- 

 ing in resonance with the continuous waves induced. 

 When receiving a signal the decrement must be adjusted so that 

 the dots and dashes of a signal only just begin to blend, as may 



Fig. 4. 



Record of signals from an Itali.in station, made in Leyden. The decrement 



of the string has been adjusted so that the dots and dashes of a signal 



only just begin to blend. 



