diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index f1c62df..b445f21 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -48,7 +48,13 @@ arma::vec PenningTrap::external_E_field(arma::vec r)
 
 arma::vec PenningTrap::external_B_field(arma::vec r)
 {
+    double x = r(0), y = r(1);
+    arma::vec::fixed<3> res;
+    res(0) = y*this->B_0;
+    res(1) = -x*this->B_0;
+    res(2) = 0.;
 
+    return res;
 }
 
 arma::vec PenningTrap::force_on_particle(int i, int j)