diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index 2ebc667..6dde7ee 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -51,11 +51,10 @@ 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.;
+    res(0) = 0.;
+    res(1) = 0.;
+    res(2) = this->B_0;
 
     return res;
 }