1902.] Waves, employed as a Receiver for Space Telegraphy. 343 



nected to the telephone, has consisted of similar wire, and I have been 

 in the habit of employing a sufficient number of turns of it to give a 

 resistance about equal to that of the telephone used. 



It would, no doubt, be possible to obtain the signals by causing the 

 iron core to act directly on a telephone diaphragm, and in this case the 

 secondary winding on the core could be omitted. The length of the 

 electric waves used in the experiments between St. Catherine's Point 

 and North Haven was about 200 metres. If longer waves are employed, 

 it is desirable that the length of the winding nearest the iron should be 

 increased. 



This detector, as I have already stated, appears to be more sensitive 

 and reliable than a coherer, nor does it require any of the adjustments 

 or precautions which are necessary for the good working of the latter. 



Further advantages in its use become apparent when it is employed 

 in connection with my syntonic system of space telegraphy. According 

 to this system, electrical syntony between the transmitter and receiver 

 is dependent on the proper electrical resonance of the various circuits 

 of transformers used in the receivers. With certain coherers one diffi- 

 culty has been that it was not always possible to restore them by 

 mechanical tapping to the same electrical resistance which they pos- 

 sessed before being affected by the transmitted electric waves, the result 

 being that the secondaries of the receiving transformers were at certain 

 times open and at other times closed by a variable resistance, thus 

 causing an appreciable variation in their natural period of electrical 

 oscillation. 



The magnetic detector which I have described possesses, on the other 

 hand, a practically uniform and constant resistance much lower than 

 that of a coherer in its sensitive condition, and, as it will work with a 

 much lower E.M.F., the secondaries of the tuning transformers can be 

 made to possess much less inductance, their period of oscillation being 

 regulated by a condenser in circuit with them, which condenser may 

 be much larger (in consequence of the smaller inductance of the circuit) 

 than those used for the same period of oscillation in a coherer circuit, 

 with the result that the receiving circuits can be tuned much more 

 accurately to a particular radiator of fairly persistent electric waves. 



The considerations which led me to the construction of the above- 

 described detector are the following : — It is a well-known fact that 

 after any change has taken place in the magnetic force acting on a 

 piece of iron, some time elapses before the corresponding change in 

 the magnetic state of the iron is complete. If the applied magnetic 

 force be either subjected to a gradual increase followed by an equally 

 gradual diminution, or caused to effect a cyclic variation, the corre- 

 sponding induced magnetic variation in the iron will lag behind the 

 changes in the applied force. To this tendency to lag behind, Pro- 

 fessor Ewing has given the name of Magnetic Hysteresis. 



