Variable resistors and potentiometers each augment the standard resistor symbol with an arrow. The variable resistor remains a two-terminal device, so the arrow is just laid diagonally across the middle. A potentiometer is a three-terminal device, so the arrow becomes the third terminal (the wiper).
On all but the simplest PCBs, wires must be printed on more than one surface of fiberglass to allow for all the required component interconnections. Each surface containing printed wires is called a layer. In a relatively simple PCB that requires only two layers, only one piece of fiberglass is required since wires can be printed on both sides. In a more complex PCB where several layers are required, individual circuit boards are manufactured separately and then laminated together to form one multi-layer circuit board. To connect wires on two or more layers, small holes called vias are drilled through the wires and fiberglass board at the point where the wires on the different layers cross. The interior surface of these holes is coated with metal so that electric current can flow through the vias. Most Digilent boards are simple four or six layer boards; some more complex computer circuit boards have more than 20 layers.
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