A new type of device is presented that allows direct measurement of spin selectivity in charge transfer processes occurring in adsorbed molecules. The new device provides direct information about the nature of the charge being transferred (electrons or holes) and on spin selectivity, if it exists. Here the device is applied for establishing the spin-dependent electron transfer through double-stranded DNA and its variation with the length of the oligomer. The DNA is self-assembled on a silver substrate and is measured under ambient conditions. The device is based on monitoring the electric potential between a ferromagnetic Ni layer and a silver layer, on top of which the DNA is self-assembled. When a dye molecule, attached to the DNA, is photoexcited, a charge transfer between the dye and the silver substrate takes place, resulting in a change in the electric potential between the Ni and the silver. If the charge transfer is spin selective, the electric potential measured depends on the direction of magnetization of the Ni.