diff --git a/crates/coremem/src/stim.rs b/crates/coremem/src/stim.rs
index 4b18cb8..8b41010 100644
--- a/crates/coremem/src/stim.rs
+++ b/crates/coremem/src/stim.rs
@@ -1,6 +1,6 @@
 use crate::real::*;
 use crate::cross::vec::Vec3;
-use crate::geom::{Meters, Region, Torus};
+use crate::geom::{HasCrossSection as _, Meters, Region, Torus};
 use rand;
 
 /// field densities
@@ -162,12 +162,8 @@ impl<R, T> CurlStimulus<R, T> {
 impl<T: TimeVarying1 + Sync> AbstractStimulus for CurlStimulus<Torus, T> {
     fn at(&self, t_sec: f32, pos: Meters) -> Fields {
         if self.region.contains(pos) {
-            let center = self.region.center();
-            let axis = self.region.axis();
             let FieldMags { e, h } = self.stim.at(t_sec);
-            let from_center_to_point = *pos - *center;
-            // TODO: is this inverted?
-            let rotational = from_center_to_point.cross(*axis);
+            let rotational = self.region.cross_section_normal(pos).norm();
             let impulse_e = rotational.with_mag(e.cast()).unwrap_or_default();
             let impulse_h = rotational.with_mag(h.cast()).unwrap_or_default();
             Fields {