The current through a conductor element is proportional to the potential difference appiled between its end provided the temperature remains constant. LIMITATIONS :1.V depends on I non-linearly. 2. The relationship between V and I depends on the sign of V for the same absolute value of V. 3. The relation between V and I is non-unique, that is, for the same current I, there is more than one value of voltage V.A vacuum tube is used to convert high voltage alternating current to direct current.