N = 10;
NN = N*N;
G = sparse(NN, NN);

node = 0;
for row=1:N;
    for col=1:N;
	node++;
	if row > 1
	    G(node, node)++;
	    G(node, node - N) = -1;
	end
	if row < N;
	    G(node, node)++;
	    G(node, node + N) = -1;
	end
	if col > 1
	    G(node, node)++;
	    G(node, node - 1) = -1;
	end
	if col < N;
	    G(node, node)++;
	    G(node, node + 1) = -1;
	end
    end
end

current = sparse(NN, 1);

Ar = 2; Ac = 2; A = Ac + N*( Ar - 1 );
Br = 7; Bc = 8; B = Bc + N*( Br - 1 );
current( A ) = -1;
current( B ) = +1;

voltage = G \ current;

VA = voltage( A );
VB = voltage( B );

full( abs( VA - VB ) )