206 Mr. G. Marconi [June 13, 



it may be found possible to receive wireless telegraph messages at a 

 speed of several hundred words a minute. At present, by means of 

 this detector, it is possible to read about 30 words a minute. 



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 caused to effect a cyclic variation, the corresponding induced 

 magnetic variation in the iron will lag behind the changes in the 

 applied force. To this tendency to lag behind Prof. Ewing has given 

 the name of magnetic hysteresis. It has been shown also by Profs. 

 Gerosa, Finzi and others, that the effect of alternating currents or 

 high-frequency electrical oscillations acting upon iron is to reduce 

 considerably the effects of magnetic hysteresis, causing the metal to 

 respond readily to any influence which may tend to alter its mag- 

 netic condition. The effect of electrical oscillations probably is to 

 bring about a momentary release of the molecules of iron from the 

 constraint in which they are ordinarily held, diminishing their 

 retentiveness and consequently decreasing the lag in the magnetic 

 variation taking place in the iron. I therefore anticipated that the 

 group of electrical waves emitted by each spark of a Hertzian radia- 

 tor would, if caused to act upon a piece of iron which is being 

 subjected at the same time to a slowly varying magnetic force, would 

 produce sudden variations in its magnetic hysteresis, which would 

 cause others of a sudden or jerky nature in its magnetic condition. 

 In other words, the magnetisation of the iron, instead of slowly follow- 

 ing the variations of the magnetic force applied, gives a sort of 

 jump each time it is affected by the electric waves emitted by each 

 spark of the radiator. These jerks in the magnetic condition of the 

 iron would, I thought, cause induced currents in a coil of wire of 

 strength sufficient to allow the signals transmitted to be detected 

 intelligibly on a telephone, or perhaps even read on a mirror galvano- 

 meter. The results obtained go to confirm my belief that this 

 detector can be advantageously substituted for the coherer for the 

 purposes of long-distance space telegraphy. 



During the last few years the developments in the practical appli- 

 cations of my system have been exceedingly rapid. Time does not 

 allow me to give you an account of the many cases in which it has 

 proved its usefulness, but it may be sufficient if I mention that Lloyd's 

 have adopted the system exclusively for use at their stations at home 

 and abroad for a period of 14 years, and that no less than 17 liners 

 plying across the Atlantic carry permanent installations. In more 

 than one case recorded in the daily papers the system has been of 

 service to vessels in distress, especially in the English Channel. No 

 less than 40 land stations (most of which are controlled by the 

 corporation of Lloyd's) are being equipped with the system in Great 

 Britain and Europe, and over 40 vessels in H.M. Navy carry installa- 

 tions. The adoption of my system in the Royal Navy has brought 



