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.