480 



Popular Science Monthly 



High Frequency Tesla Coil for 

 Electrical Experimenters 



ANY one who owns a wireless equipment 

 L has the apparatus necessary to operate 

 this coil. It should give at least a 6- in. 

 spark under favorable conditions. The 

 drawing gives the necessary dimensions and 

 shows parts cut away, in order to make the 

 construction clearer. The only apparatus 

 necessary is a spark-gap, an induction coil 

 or transformer and plate-condenser. 



INO. COIL 



Detail of the high frequency coil and the dia- 

 gram showing how to connect the apparatus 



The primary winding consists of 22 turns 

 of No. 14 rubber-covered copper wire 

 wound on a fiber tube, 6 in. outside diameter, 

 and of a length that will hold the 22 turns 

 of wire. The secondary coil is made up of 

 one layer of No. 34 double silk-covered 

 wire, wound on a tube 3 in. outside diame- 

 ter and 12 in. long. The tall posts carrying 

 the high potential spark-gap should be at 

 least 18 in. high. Fasten the parts to- 



gether with glue, and do not use any nails 

 or screws except for fastening parts to the 

 base. Finish all over with two or three 

 good coats of thin shellac, drying well 

 between coats. 



It is well known that a spark drawn from 

 a coil of this type is not dangerous and only 

 a slight burning sensation is experienced 

 when it passes to the body. Sparks may 

 be drawn from any part of the body with- 

 out discomfort to the operator, but to the 

 astonishment of the onlookers. This coil 

 may be used quite satisfactorily for enter- 

 tainments or for various high-frequency 

 experiments. The diagram shows how to 

 connect the apparatus. 



Testing Wires and Battery Terminals 

 for Polarity with Litmus Paper 



POLARITY indicators are expensive 

 instruments and the testing of a few 

 wires would not warrant the purchase of 

 one. One simple test method utilizes 

 litmus paper. Procure from a local drug 

 store some red litmus paper and thoroughly 

 soak it in a solution made up of one table- 

 spoonful of salt to a glass of water. When 

 it is thoroughly soaked remove the paper 

 carefully so as not to tear it, and hang up 

 to dry where nothing will touch it except 

 the hangers. Never dry it between blotters. 



These test papers are used as follows: 

 Take a piece of the paper about ij^ in. 

 long by }/2 ifL. wide and slightly moisten it 

 and place the ends of the wire to be tested 

 on the paper so that they will be about 

 ^ in. apart. A deep red spot will appear 

 on the paper at the end of one of the wires 

 and a blue spot at the other. The red 

 designates the positive and the blue the 

 negative. Sometimes the red spot will not 

 show owing to the low potential difference 

 between the wires, but the blue will always 

 appear, designating the negative wire. 



There are two kinds of litmus paper, red 

 and blue. If the red cannot be obtained 

 get the blue, but the potential difference 

 between the wires will make the red show 

 up more in this instance. After using this 

 paper it can be dried as when taken from 

 the salt solution, and used again. 



The Editor of the Popular Science Monthly is always glad to hear from readers 

 who have made simple and useful things for the home and the shop with their own 

 hands and who would like to tell others of their own success. Articles from amateur 

 mechanics, electricians and wireless operators are paid for on acceptance, promptly and 

 liberally. But contributors must understand that only unpublished contributions, 

 offered exclusively to the POPULAR SCIENCE MONTHLY, are desired. 



