394 



THE WORLD'S ADVANCE 



Under this condition it would be 

 thought that no energy would be trans- 

 ferred to the detector circuit, but, as a 

 matter of fact, the opposite is true. This 

 sounds far fetched, but here is an anal- 

 ogy: 



Every one has read of Tesla's experi- 

 ment where he puts a heavy ring of cop- 

 per in a high-frequency field. What 

 happens? He induces so much current 

 into that ring that it grows red hot and, 

 if continued long enough, it melts. 



Now, returning again to the circuit in 

 the Cohen tuner, the question is asked: 

 "Can you induce a current in a circuit 

 possessing no capacity or inductance?" 

 The answer is yes. 



Following up this line of reasoning, it 

 will be noticed that if we could do away 

 with the effect of inductances in the in- 

 termediate circuit we could remove the 

 capacities likewise. It is, of course, im- 

 possible to entirely remove the induct- 

 ances, but we can reduce them to such a 

 small amount that their effects on the 

 circuit will not be noticeable to any great 

 degree. 



This is done by using two or three, 

 no more, turns of stranded wire for each 

 coil in the linking circuit and keeping 

 the leads between them as short and 

 straight as possible. 



In Fig. 2 are shown the hook-up and a 

 front view of a cabinet incorporating the 

 link circuit. Of course, the coils of the 

 aerial and detector circuit should be so 

 placed as to have a minimum amount of 

 direct inductive effect on each other and 

 this can be obtained by placing them some 

 distance apart or by placing their axes 

 at right angles to each other, as is done 

 in this case to make it as compact as 

 possible. 



The box is made of % inch walnut 

 or mahogany and fastened together with 

 brass screws. The dimensions are op- 

 tional with the builder. Holes are drilled 

 in the front to accommodate the two ten- 

 point switches and the small two-point 

 switch as well as the binding posts for 

 the 'phones shown near the bottom. 



Each end is also drilled for a ten- 

 point switch. The switches on the ends 

 have every other turn connected to their 

 contacts, while those on the front have 

 twenty turns between contacts. 



The coils are similar and wound on 

 cardboard or fibre 6 inches long and 4 

 inches outside diameter. They are 

 wound with No. 24 enameled wire topped 

 every other turn for the first twenty 

 turns and then every twenty turns till 

 200 more turns have been wound on. 

 The taps from the coil mounted on the 

 top of the box should be taken down the 

 inside of the tube, while those from the 

 coil inside the case can be run direct to 

 the switches. 



The right-hand switches control the 

 primary and the other two are for the 

 secondary adjustment. The aerial and 

 ground binding posts can be mounted 

 anywhere the builder decides. 



The link circuit consists of three turns 

 taken around each coil, using No. 18 

 stranded copper wire; silk covered flex- 

 ible cord is the best. These coils are 

 wound around the end of the coil from 

 which the top is taken at every other turn 

 so as to always be in the active field 

 of the coil. 



The type of detector is immaterial, but 

 the shorting and selecting switch deserves 

 special attention. The two contacts of 

 this switch are mounted very close to- 

 gether, so close that the switch blade can 

 bridge the two. Now referring to the 

 hook-up, it will be seen that the detect- 

 ors are really in series and the switch 

 shorts the one not in use, but when the 

 blade bridges both contacts both detect- 

 ors are shorted. This method saves the 

 expense and trouble of having a sepa- 

 rate switch to short the detectors when 

 sending. 



In using this set the switches on the 

 front are adjusted first for coarse tuning 

 and when the signals are the loudest the 

 other two are used for fine tuning. 



The selectivity is high, due to the link- 

 ing circuit having practically no damp- 

 ing and though not capable of sustaining 

 oscillations it will not damp them out, 

 thus enabling sharp tuning and the elim- 

 ination of stray currents and static. 



The set described will tune to about 

 600 meters or higher if variable conden- 

 sers are added in parallel to the windings. 

 In the latter case the two switches on the 

 ends may be done away with and the 

 two variable condensers inserted into the 

 open ends of the tube inside. 



