5 U.S.C. § 5703

Per diem, travel, and transportation expenses; experts and consultants; individuals serving without pay

An employee serving intermittently in the Government service as an expert or consultant and paid on a daily when-actually-employed basis, or serving without pay or at $1 a year, may be allowed travel or transportation expenses, under this subchapter, while away from his home or regular place of business and at the place of employment or service.

( Pub. L. 89–554 , Sept. 6, 1966 , 80 Stat. 499 ; Pub. L. 91–114, § 2 , Nov. 10, 1969 , 83 Stat. 190 ; Pub. L. 94–22, § 4 , May 19, 1975 , 89 Stat. 85 .)