Explain the secondary standard of emf (Westoncell) with necessary diagram

(b) (i) Explain the secondary standard of emf (Westoncell) with necessary diagram. (8)
(ii) Classify and explain the various types of errors. (8)
12. (a) (i) Explain the principle of dual slope integrating type digital voltmeter with neat diagram.
(10)
(ii) Compare and contrast the analog and digital meters.