The principal types of electrodes are : NUMBER@1 -RRB- simple solid conductors , such as discs and needles -LRB- singles or arrays -RRB- , NUMBER@2 -RRB- tracings on printed circuit boards , and NUMBER@3 -RRB- hollow tubes filled with an electrolyte , such as glass pipettes .

The principle types of electrodes are : NUMBER@1 -RRB- simple solid conductors , such as discs and needles -LRB- singles or arrays -RRB- , NUMBER@2 -RRB- tracings on a printed circuit boards , and NUMBER@3 -RRB- hollow tubes filled with an electrolyte , such as glass pippettes .
