The principle types of electrodes are : 1 ) simple solid conductors , such as discs and needles ( singles or arrays ) , 2 ) tracings on a printed circuit boards , and 3 ) hollow tubes filled with an electrolyte , such as glass pippettes . The principal types of electrodes are : 1 ) simple solid conductors , such as discs and needles ( singles or arrays , often insulated except for the tip ) , 2 ) tracings on printed circuit boards , also insulated except for the tip , and 3 ) hollow tubes filled with an electrolyte , such as glass pipettes filled with potassium chloride solution or another electrolyte solution .