674 Prof. Fleming and Mr. Dyke on the Production of 



this manner, a succession of Morse signals may be sent, or 

 long and short signals of any kind for the purpose of testing 

 the transmission of any particular words or letters. If the 

 spark-chamber is made of thick cast-iron the apparatus will be 

 nearly noiseless, and may therefore be set up in a Laboratory 

 without disturbing other workers, which is not the case when 

 an open oscillatory spark is employed. At a distance, say of 

 50 to 150 feet or more, another square circuit may be set up 

 consisting of a similar square coil of insulated wire L 2 (see 

 fig. 2) and a condenser C 2 , which is preferably a condenser of 



Fig. 2.— Receiving Circuits, 

 variable capacity for tuning purposes. A convenient form is 

 one consisting of fixed semicircular plates and a number of 

 movable semicircular plates fixed on a shaft, which can be 

 rotated, so as to bring the second set of plates more or less in 

 between the first set, the vessel being filled with a highly 

 insulating oil. Two of such closed oscillatory circuits can be 

 set up at a distance, say of 50, 100, or 200 feet within a large 

 building, and even the interposition of brick walls makes no 

 difference, provided they do not contain metal girders. 



To test, then, a radiotele graphic detector of any kind, it is 

 necessary to be certain that the detector per se, when un- 

 connected to the oscillatory receiving-circuit, is not directly 

 affected by the spark at the distance at which the sending and 

 receiving circuits are set up ; but this can easily be done, and 

 then any particular type of* oscillation-detector, D (see fig. 2) , 

 whether of the current actuated type or the potential actuated 

 type, can be tested as to sensibility by inserting it, either in 

 series with the condenser of the receiving-circuit, or in parallel 

 with the condenser of the receiving circuit. The detector D 

 is associated with a telephone T and a battery B shunted by 

 resistance R as usual. The use of the Closed circuits has this 

 great advantage, that being directive radiators and absorbers, 

 it is possible, by a displacement of the planes of the magnetic 



