Several tubes may be arranged in a rack as shown in Figure 225.
A helix consists of a spiral of brass ribbon set in a wooden frame. The two strips composing the frame are each nine inches long. The spiral consists of eight turns of brass ribbon, three-eighths of an inch wide, set in saw-cuts made in the frame. A binding-post is connected to the outside end of the ribbon.
Figure 228 shows how to connect a helix and a condenser to a coil and a spark-gap.
The two clips are made by bending a strip of sheet brass and connecting a piece of flexible wire to one end.
Fig. 224.—A Test-Tube Leyden Jar.
In large stations, the best position for the clips is found by placing a "hot-wire ammeter" in the aerial circuit and then moving the clips until the meter shows the highest reading.
The young experimenter will have to tune his set by moving the helix clips about until the best results are obtained in sending.
Fig. 225.—Eight Test-Tube Leyden Jars mounted in a Wooden Rack.