262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



Thus, however much one clnini gets out of step with the other, the 

 fault is limited to each revolution, and both drums must always start 

 off in unison for each new revolution. I have found that where each 

 operator endeavors to keep his motor running uniformly by regu- 

 lating the resistance according to the fluctuations recorded by the 

 frequency meter, the personal element makes itself visible in the re- 

 sults; straight lines appear wavy, and the synchronism is not at all 

 good. I therefore tried very carefully calibrating the motors by 

 timing first, and then arranged that, once started, the motors should 

 not be touched ; the gain in speed of each is approximately the same 

 if both motors are run from secondary batteries of the same ampere- 

 hour capacity, and in this way we have obtained the most perfect re- 

 sults as regards synchronization. 



The great advantage of this process is that the whole operation is 

 in full view, whereas with systems in which the received picture is 

 obtained on a photographic film one has to develop such film before 

 it is possible to discover whether anything is wrong. With the re- 

 ceiver described, the operator keeps his hand on the sliding contact 

 of the resistances, and merely adjusts their position during the first 

 two or three seconds, according to the condition of the electrolytic 

 marks, i. e., whether crisp and concise or not. Tlie transmitting cyl- 

 inder can be used as the receiving cylinder, and the apparatus is thus 

 reduced to the limits of simplicity. 



Toward the end of last year I designed a portable machine, two 

 of which Mr. Sanger-Shepherd has just completed, embodying in 

 them a number of improvements of his own, and these machines, 

 which have worked successfully on their trials, are shoAvn on the 

 lecture table to-night. They are suitable for line or wireless work, 

 and will, I believe, prove of great value in naval and military 

 operations. 



The Daily Mirror inaugurated the Paris-London photographic 

 service in November, 1907, with Prof. Korn's selenium instruments, 

 which I shall briefly describe, as Korn is now making two new 

 selenium apparatus wath the view of transmitting photographs from 

 New York to London. In this system use is made of the fact that 

 the electrical resistance of the metal selenium varies according to 

 the strength of illumination to which it is subjected, a beam of light 

 passed through the light and dark parts of a photograph in succes- 

 sion being used to vary the strength of an electric current sent to the 

 receiving apparatus. 



In Korn's selenium transmitter light is concentrated from a Nernst 

 lamp to pass through a revolving glass cylinder, round which a trans- 

 parent photograph (printed on celluloid) is fixed, the beam travers- 

 ing the film at its brightest part, where the rays come to a focus 

 (fig. 2). The light which passes through the picture is reflected by a 



