Article 1, Section 3, Clauses 4 and 5

The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided.

The Senate shall chuse their other Officers, and also a President pro tempore, in the Absence of the Vice President, or when he shall exercise the Office of President of the United States.

1.Records of the Federal Convention
2.A Native of Virginia, Observations upon the Proposed Plan of Federal Government, 1788
3.Debate in North Carolina Ratifying Convention, 24--25 July 1788
4.Joseph Story, Commentaries on the Constitution 2:§§ 732--33, 735--39, 1833