1. An apparatus for reading a current signal from a source in an array of sources, each current having a changeable sign, a small amplitude and several orders of magnitude dynamic range comprising: a first conductor; a second conductor means for carrying a first addressing signal; coupling means for coupling said current signal to said first conductor upon receipt of said first addressing signal; an output node; compression means having an input for coupling to a source of a reference voltage and coupled to said current signal only through said first conductor and coupled to said output node for converting said current signal to an output signal at said output node by a transfer function relating the voltage difference between the voltage at said output node and said reference voltage to said current signal such that said current signal increases faster than linearly in the positive direction for positive voltage differences and increases faster than linearly in the negative direction for negative voltage differences.