def get_delta(vmag_matrix, deriv_step):
    """ Convert voltage mag. data into the change in voltage mag. wrt time.  
        
    This function finds the change in voltage magntiude between 
    time-points. The spacing between time-points is a variable that can be
    changed. The change in voltage magnitude is calculated as Delta_Vmag =
    Vmag(node x, time t) - Vmag(node x, time t-deriv_step).
        
    Args
    -------
    vmag_matrix: 2D ndarray, shape (# of data-points, # of nodes)
        This is the voltage magnitude data at different nodes. We want to 
        transform this data so we end up with the change
        in voltage magnitude from one time-point to the next for each node.
        
    deriv_step: int 
        This parameter decides how many time-points for which
        to compute the voltage magntiude difference at a node. Ex. if
        deriv_step = 5, then the change in voltage magnitude will be
        Delta_Vmag = Vmag(node x, time t) - Vmag(node x, time t-5).
        
    Returns
    -------
    vmag_matrix: 2D ndarray, shape (# of data-points, # of nodes)
        This matrix now represents the change in voltage
        magnitude at a node from one time-point to the next. Each column
        corresponds to a different node and each row to a time-point.
    """
    end = len(vmag_matrix[:, (1)]) - 1
    vmag_matrix = vmag_matrix[deriv_step + 1:-1, :] - vmag_matrix[1:end -
        deriv_step, :]
    return vmag_matrix
