288 



Popular Science Monthly 



pendulum can be adjusted to swing with a 

 definite period of one second, or of two seconds, 

 by changing the length of the string or rod, and 

 in that way only. 



Copper-Plating Leyden Jars 



D. A. S., Midland Beach, Staten Island, 

 N. Y., writes: 



Q. I. Can you inform me how I may prepare 

 glass in order that the surface may be copper- 

 plated? How can I prevent blistering? I hear 

 much these days of the copper-plated Leyden 

 jar. 



A. I. The process is rather e.xpensive. One 

 manufacturer coats the bare glass with a cold 

 silver solution such as is applied to the rear of 

 mirrors. The jar is then placed in a furnace and 

 heated near to the melting point whereupon the 

 silver is thoroughly burned into the glass. The 

 jar is then allowed to cool slowly and afterward 

 placed in an ordinary electroplating vat and 

 given a heavj' coating of copper. 



Another manufacturer applies a cold silver 

 solution to the inside of the glass and a coating 

 of graphite held in place by shellac to the outside 

 of the glass. The jar is placed in the electro- 

 plating bath without burning. 



Dimensions for Transmitting 

 Transformer 



A. W., Fayette, Ala., writes: 



Q. I. Please give me the dimensions of the 

 primary and secondary windings for an oscillation 

 transformer for use with a I K.W. transmitting 

 set. Please state the diameter of both windings. 



A. I. It is assumed that you desire an 

 oscillation transformer of the pancake type, and 

 if so, it should have the follow-ing dimensions: 

 The primary winding has eight turns of flat 

 copper ribbon placed on an insulating support 

 edgewise, the copper being about \ in. in width 

 by 1/16 in. in thickness. The outside diameter 

 of the winding is 10 ins. and the inside diameter 

 about 4j ins.; the turns are therefore placed 

 about J in. apart. The secondary winding of 

 the oscillation transformer may consist of about 

 18 turns of the same size ribbon, also spaced 

 \ in. apart. The outside diameter of this winding 

 is 14 ins. and the inside diameter about \\ ins. 



Q. 2. Should the ribbon be wound in the 

 same direction in both windings? 



A. 2. It makes no difference which way they 

 are wound. 



Q. 3. In the construction of a condenser for 

 a I K.W. transformer, should the tinfoil be 

 placed on both sides of the glass plates and the 

 plates then stacked together so that the tinfoil 

 of one plate touches the tinfoil of the other, and 

 should the copper ribbon for making contact 

 ■with the tinfoil be brought out l)etween the 

 plates, and should the odd number of plates be 



connected to the right condenser terminal and 

 the even numbers to the left, or vice versa? 

 Would it be satisfactory to place the tinfoil on 

 a glass plate, then lay on top of it another glass 

 plate, followed by a second sheet of tinfoil, and 

 so on throughout the series until complete? 



A. 3. It is preferable to coat both sides with 

 tinfoil and the best method is to place the foil 

 on the glass plates with a good grade of fish glue, 

 after which the plate is shellacked to prevent the 

 foil from blistering. Alternate plates are con- 

 nected to one terminal, and the inter\'ening plates 

 to the inside terminal. The surfaces of the sheets 

 of tinfoil from each plate should be pressed tight 

 against the sheet ne.xt to it. The reason for this 

 is that unless the tinfoil is firmly attached to 

 the plates, a violent brush discharge takes place 

 between the glass and the foil, causing blisters. 



Q. 4. What should be the capacity in 

 amperes for a transmitting key to break the 

 primary circuit of a l-K.W. Iio-volt 6o-cycle 

 transformer? 



A. 4. Owing to phase displacement in the 

 primary circuits an ammeter will indicate from 

 14 to 16 amperes, but of course with no phase 

 displacement the ammeter should read about 

 9j amperes. The transmitting key should have 

 a current-carrying capacity of 15 amperes. 



Radio Frequency Changers 

 W. H. H., Cold Spring, N. J., asks: 

 Q. I. Will you kindly give me a more 

 complete description of the transformer which 

 doubles the frequency of the current imposed on 

 the primary winding, which you mention in the 

 article entitled "Long Distance Wireless Teleg- 

 raphy" in the August IQ15 issue of this publi- 

 cation. 



A. I. It would be far too long an article to 

 give in this column, but you w-ill find one of the 

 most complete short descriptions of radio 

 frequency changers in print in the March 1915, 

 Volume 3, No. i issue of the Proceedings of 

 The Institute of Radio Engineers. This is a 

 35-page article and it covers not only static but 

 dynamic, electrostatic as well as electromagnetic, 

 frequency changers. It is fully illustrated and 

 contains many graphs showing the relations of 

 the several currents during the frequency 

 changers. If you are at all interested in the sub- 

 ject, we most certainly recommend that you 

 read this article. If you cannot obtain a copy 

 of this publication from a library or member of 

 the Institute you can purchase a copy for fl 

 direct from the Institute Secretary, ill Broad- 

 way, New York. 



Q. 2. To what degree is the core of the 

 transformer saturated by the direct current? 



A. 2. The core is saturated to the point 

 known as the knee of the curve. It is the point 

 where the saturation begins to increase very 

 slowly with increase of magnetizing current. 



