For each motor tractor used on the highways of this state there shall be paid a license or privilege tax of $100; provided, however, that this license shall not be collected for a tractor when run on a highway to be transferred from one point to another for use on a farm with or without a “small trailer” or with or without a “semitrailer,” or when used on the highway for transferring what is commonly known as a “portable sawmill” or a “well-boring outfit,” or when used on the highway by a farmer for the purpose of transporting to and from his farm farm products or products to be used on his farm.