In general no formality is necessary for the appointment of an agent to contract on behalf of his principal. Authority may be given by expression of assent thereto in any way. Frequently the principal and agent make a contract providing that the agent shall act as such. For the formation of such a contract the same rules are applicable, as to any other contract; but a contract is not necessary in order to give an agent authority. The principal's authorization may neither expressly nor impliedly request any expression of assent by the agent as a condition of the authority, and in such a case any exercise of the authority by the agent during the term for which it was given or within a reasonable time, if no fixed term was mentioned, will bind the principal.1