RADIO COMMUNICATION 261 



ing station from which the signals are desired. When one tuning 

 coil is used with two sliders in this manner, and so becomes a 

 part of two circuits, it is known as an auto-transformer. In tun- 

 ing such an outfit the primary circuit or open oscillating circuit 

 must be tuned to respond to the frequency of the sending sta- 

 tion, and then the secondary or closed oscillating circuit must be 

 tuned to the primary circuit. Sometimes a small fixed condenser 

 is shunted across the phones, permitting the vibrations to flow 

 more easily. This is known as the phone condenser, and has a 

 capacity usually of about .001 microfarads. This condenser 

 (re) is sometimes left out of the circuit, and in such case the 

 phone cords themselves act as a condenser. 



A simple receiving set such as the one described, but equipped 

 with a simple tuning coil, may easily be made and assembled 

 as follows. For the tuning coil procure a cardboard tube about 

 five or six inches long and three and a half to four inches in 

 diameter. Round cardboard oatmeal boxes serve this purpose 

 very well. This tube is to be wound with insulated wire and 

 mounted horizontally on a board, which may serve also as the 

 base for the detector and the terminals for the phone connection. 



For the base secure a piece of material, wood or fiber, about 

 two inches wider than the diameter of the tube and four or five 

 inches longer than the tube. The tube is provided with end 

 pieces of the same material as the base. Tliese end blocks are 

 to be cut one inch less in width than the base board, of a height 

 equal to the width of the base board. In each end piece is now 

 cut a shallow groove to receive the ends of the cardboard tube. 

 This groove should be the same distance from the top of the end 

 piece as from the sides. The guides for the sliders and the sliders 

 themselves for the tuning coil may be secured cheaply at any 

 radio supply shop or ten-cent store. The guide rods should be of 

 square metal material of f - to J-inch stuff. They must be as long 

 as the length of the tube and end pieces when the tube is fitted 

 into the grooves in the end pieces. The sliders are small blocks 

 of wood with a square notch cut on one side to fit snugly over the 



