On the other hand you can wind with single silk covered wire and bring out taps at the 0, 2, 4, 6, 8, 10, 14, 20, 28, 36, 44, 56, 66, and 75th turns. To make a tap drill a small hole through the tube, bend the wire into a loop about a foot long and pull this loop through the hole as shown in Fig. 110. Then give the wire a twist, as shown, so that it can't pull out, and proceed with your winding. Use 26 s. s. c. wire. You will need about 80 feet and might buy 200 to have enough for the secondary coil. Make contacts to the taps by two rotary switches as shown in Fig. 112. You can buy switch arms and contacts studs or a complete switch mounted on a small panel of some insulating compound. Let switch _s_{1}_ make the contacts for taps between 14 and 75 turns, and let switch _s_{2}_ make the other contacts. For the secondary coil use the same size of wire and of core. Wind 60 turns, bringing out a tap at the middle. To tune the secondary circuit you will need a variable condenser. You can buy one of the small ones with a maximum capacity of about 0.0003 mf., one of the larger ones with a maximum capacity of 0.0005 mf., or even the larger size which has a maximum capacity of 0.001 mf. I should prefer the one of 0.0005 mf. You will need a crystal detector--I should try galena first--and a so-called "cat's whisker" with which to make contact with the galena. For these parts and for the switch mentioned above you can shop around to advantage. For telephone receivers I would buy a really good pair with a resistance of about 2500 ohms. Buy also a small mica condenser of 0.002 mf. for a blocking condenser. Your entire outfit will then look as in Fig. 112. The switch _S_ is a small knife switch. To operate, leave the switch _S_ open, place the primary and secondary coils near together as in the figure and listen. The tuning is varied, while you listen, by moving the slider of the slide-wire tuner or by moving the switches if you have connected your coil for that method. Make large changes in the tuning by varying the switch _s_{1}_ and then turn slowly through all positions of _s_{2}_, listening at each position. [Illustration: Fig 112]