Cross product error in plotting spiral?

I have the following code

I = .017; %Amps
Hx = 0;
Hy = 0;
H = 0;

Px = 0;
Py = 0;
Pz = 1.65;

t = 0:3500;
u = .001857;
r0 = 10;
r = r0 +u*t;

omega = pi/250;
phi0 = 3*pi/2;
phi = -omega*t+phi0;
x = r .* cos(phi);
y = r .* sin(phi);
x = x - mean(x)
y = y - mean(y)
plot(x,y)
grid on;

dLx = diff(x);
dLy = diff(y);

dL = [dLx dLy 0];
aR = [Px-x Py-y Pz];
R = sqrt((Px-x).^2+(Py-y).^2+Pz.^2);


H = H + I*(cross(dL,aR))/(4*pi*R^2);

and I want to follow the following equation

but I get the follwoing error when run.

Error using cross (line 25)

A and B must be of length 3 in the dimension in which the cross product is taken.

Error in Project_2 (line 33)

H = H + I*(cross(dL,aR))/(4*pi*R^2);

Any ideas or help would be helpful

Answer 1

[KSSV]

I = .017; %Amps
Hx = 0;
Hy = 0;
H = 0;
Px = 0;
Py = 0;
Pz = 1.65;
t = 0:3500;
u = .001857;
r0 = 10;
r = r0 +u*t;
omega = pi/250;
phi0 = 3*pi/2;
phi = -omega*t+phi0;
x = r .* cos(phi);
y = r .* sin(phi);
x = x - mean(x);
y = y - mean(y);
plot(x,y)
grid on;
dLx = gradient(x');
dLy = gradient(y');
dL = [dLx dLy zeros(size(dLx))];
aR = [(Px-x)' (Py-y)' repmat(Pz,size(x))'];
R = sqrt((Px-x).^2+(Py-y).^2+Pz.^2)';

H = I*(cross(dL,aR))./(4*pi*R.^2);
H = cumsum(H) ;  % you have to pick the last one 

[Alec Lothian]

Can you explain why you used the gradient function and what the x' notation means