diff --git a/examples/toroid25d.rs b/examples/toroid25d.rs
index d76fad5..b610e9d 100644
--- a/examples/toroid25d.rs
+++ b/examples/toroid25d.rs
@@ -1,106 +1,119 @@
 use coremem::{Driver, Flt, mat, meas};
 use coremem::geom::{Coord, CylinderZ, Index, Meters, Vec2, Vec3};
 use coremem::stim::{Stimulus, Sinusoid};
+use log::trace;
 
 fn main() {
     coremem::init_logging();
-    let feat_size = 10e-6; // feature size
-    let from_m = |m| (m/feat_size) as u32;
-    let m_to_um = |px| (px * 1e6) as u32;
-    let to_m = |px| px as Flt * feat_size;
-    let width = 1000e-6;
-    let depth = 600e-6;
-    let conductor_inner_rad = 0e-6;
-    let conductor_outer_rad = 19e-6;
-    let ferro_inner_rad = 100e-6;
-    let ferro_outer_rad = 200e-6;
-    let buffer = 50e-6;
-    let peak_current = 2e3;
-    let current_duration = 1e-9; // half-wavelength of the sine wave
-    let conductivity = 1.0e5;
-    let half_width = width * 0.5;
-    let half_depth = depth * 0.5;
+    for p in 0..20 {
+        let ferro_depth = 10e-6 * (20-p) as Flt;
+        let feat_size = 10e-6; // feature size
+        let from_m = |m| (m/feat_size) as u32;
+        let m_to_um = |px| (px * 1e6) as u32;
+        let to_m = |px| px as Flt * feat_size;
+        let width = 2500e-6;
+        let depth = 200e-6;
+        let conductor_inner_rad = 0e-6;
+        let conductor_outer_rad = 19e-6;
+        let ferro_inner_rad = 100e-6;
+        let ferro_outer_rad = 200e-6;
+        //let ferro_depth = 10e-6;
+        let buffer = 250e-6;
+        let peak_current = 2e3;
+        let current_duration = 1e-9; // half-wavelength of the sine wave
+        let conductivity = 1.0e5;
+        let half_width = width * 0.5;
+        let half_depth = depth * 0.5;
 
-    let width_px = from_m(width);
-    let depth_px = from_m(depth);
-    let size_px = Index((width_px, width_px, depth_px).into());
-    let mut driver = Driver::new(size_px, feat_size);
-    driver.set_steps_per_frame(8);
-    //driver.set_steps_per_frame(40);
-    //driver.set_steps_per_frame(200);
-    driver.add_y4m_renderer(&*format!("toroid25d-flt{}-{}-feat{}um-{:.1e}A-{:.1e}s--radii{}um-{}um-{}um.y4m",
-        std::mem::size_of::<Flt>() * 8,
-        *size_px,
-        m_to_um(feat_size),
-        peak_current,
-        current_duration,
-        m_to_um(conductor_outer_rad),
-        m_to_um(ferro_inner_rad),
-        m_to_um(ferro_outer_rad),
-    ));
-    let conductor_region = CylinderZ::new(
-        Vec2::new(half_width, half_width),
-        conductor_outer_rad);
-    // driver.add_term_renderer();
-    driver.add_measurement(meas::Label(format!("Conductivity: {}, Imax: {:.2e}", conductivity, peak_current)));
-    driver.add_measurement(meas::Current(conductor_region.clone()));
-    driver.add_measurement(meas::Magnetization(
-        Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticFlux(
-        Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticStrength(
-        Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::Magnetization(
-        Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticFlux(
-        Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticStrength(
-        Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::Magnetization(
-        Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticFlux(
-        Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
-    ));
-    driver.add_measurement(meas::MagneticStrength(
-        Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
-    ));
+        let duration = 1e-9;
 
-    let center = Vec2::new(half_width as _, half_width as _);
+        let width_px = from_m(width);
+        let depth_px = from_m(depth);
+        let size_px = Index((width_px, width_px, depth_px).into());
+        let mut driver = Driver::new(size_px, feat_size);
+        // driver.set_steps_per_frame(8);
+        //driver.set_steps_per_frame(40);
+        //driver.set_steps_per_frame(80);
+        driver.set_steps_per_frame(120);
+        //driver.set_steps_per_frame(200);
+        driver.add_y4m_renderer(&*format!("toroid25d.5-flt{}-{}-feat{}um-{:.1e}A-{:.1e}s--radii{}um-{}um-{}um-{}um.y4m",
+            std::mem::size_of::<Flt>() * 8,
+            *size_px,
+            m_to_um(feat_size),
+            peak_current,
+            current_duration,
+            m_to_um(conductor_outer_rad),
+            m_to_um(ferro_inner_rad),
+            m_to_um(ferro_outer_rad),
+            m_to_um(ferro_depth),
+        ));
+        let conductor_region = CylinderZ::new(
+            Vec2::new(half_width, half_width),
+            conductor_outer_rad);
+        // driver.add_term_renderer();
+        driver.add_measurement(meas::Label(format!("Conductivity: {}, Imax: {:.2e}", conductivity, peak_current)));
+        driver.add_measurement(meas::Current(conductor_region.clone()));
+        driver.add_measurement(meas::Magnetization(
+            Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticFlux(
+            Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticStrength(
+            Meters((half_width + ferro_inner_rad + 2.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::Magnetization(
+            Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticFlux(
+            Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticStrength(
+            Meters((half_width + ferro_inner_rad + 1.0*feat_size, half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::Magnetization(
+            Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticFlux(
+            Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
+        ));
+        driver.add_measurement(meas::MagneticStrength(
+            Meters((half_width + 0.5 * (ferro_inner_rad + ferro_outer_rad), half_width, half_depth).into())
+        ));
 
-    for z_px in 0..depth_px {
-        for y_px in 0..width_px {
-            for x_px in 0..width_px { 
-                let loc = Index((x_px, y_px, z_px).into());
-                let d = Vec2::new(to_m(x_px), to_m(y_px)) - center;
-                let r = d.mag();
-                if (conductor_inner_rad as _..conductor_outer_rad as _).contains(&r) {
-                    *driver.state.get_mut(loc).mat_mut() = mat::Static::conductor(conductivity).into();
-                } else if (ferro_inner_rad as _..ferro_outer_rad as _).contains(&r) {
-                    //if (half_depth_px-5..half_depth_px+5).contains(z_px) {
-                        *driver.state.get_mut(loc).mat_mut() = mat::db::ferroxcube_3r1();
-                    //}
+        let center = Vec2::new(half_width as _, half_width as _);
+
+        for z_px in 0..depth_px {
+            for y_px in 0..width_px {
+                for x_px in 0..width_px { 
+                    let loc = Index((x_px, y_px, z_px).into());
+                    let d = Vec2::new(to_m(x_px), to_m(y_px)) - center;
+                    let r = d.mag();
+                    if (conductor_inner_rad as _..conductor_outer_rad as _).contains(&r) {
+                        *driver.state.get_mut(loc).mat_mut() = mat::Static::conductor(conductivity).into();
+                    } else if (ferro_inner_rad as _..ferro_outer_rad as _).contains(&r) {
+                        let half_depth_px = from_m(half_depth);
+                        let ferro_depth_px = from_m(ferro_depth);
+                        if (half_depth_px-ferro_depth_px/2 .. half_depth_px+(ferro_depth_px+1)/2).contains(&z_px) {
+                            trace!("placing ferro at {:?}", loc);
+                            *driver.state.get_mut(loc).mat_mut() = mat::db::ferroxcube_3r1();
+                        }
+                    }
                 }
             }
         }
-    }
-    let boundary_xy = half_width - ferro_outer_rad - buffer;
-    println!("boundary: {}um", m_to_um(boundary_xy));
-    let boundary = Index((from_m(boundary_xy), from_m(boundary_xy), 0).into());
-    driver.add_upml_boundary(boundary);
+        let boundary_xy = half_width - ferro_outer_rad - buffer;
+        println!("boundary: {}um", m_to_um(boundary_xy));
+        let boundary = Index((from_m(boundary_xy), from_m(boundary_xy), 0).into());
+        driver.add_upml_boundary(boundary);
 
-    driver.add_stimulus(Stimulus::new(
-        conductor_region.clone(),
-        Sinusoid::from_wavelength(Vec3::new(0.0, 0.0, peak_current), current_duration * 2.0
-)));
+        driver.add_stimulus(Stimulus::new(
+            conductor_region.clone(),
+            Sinusoid::from_wavelength(Vec3::new(0.0, 0.0, peak_current), current_duration * 2.0
+    )));
 
-    loop {
-        driver.step();
+        while driver.dyn_state().time() < duration {
+            driver.step();
+        }
     }
 }
diff --git a/src/driver.rs b/src/driver.rs
index 41e54a9..c9a5441 100644
--- a/src/driver.rs
+++ b/src/driver.rs
@@ -137,7 +137,7 @@ impl Driver {
                             0.0
                         };
                         let cond = Vec3::new(cond_x, cond_y, cond_z);
-                        trace!("cond: {:?}", cond);
+                        // trace!("cond: {:?}", cond);
                         let conductor = mat::Static::anisotropic_conductor(cond);
                         *self.state.get_mut(Index(Vec3u::new(x, y, z))).mat_mut() = conductor.into();
                     }
diff --git a/src/geom/units.rs b/src/geom/units.rs
index fd31d46..f204f62 100644
--- a/src/geom/units.rs
+++ b/src/geom/units.rs
@@ -9,7 +9,7 @@ pub trait Coord: Copy + Clone {
     fn from_index(other: Index, feature_size: Flt) -> Self;
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct Meters(pub Vec3);
 
 impl Coord for Meters {
@@ -34,7 +34,7 @@ impl Deref for Meters {
     }
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct Index(pub Vec3u);
 
 impl Coord for Index {
diff --git a/src/geom/vec.rs b/src/geom/vec.rs
index fab4e83..5508cac 100644
--- a/src/geom/vec.rs
+++ b/src/geom/vec.rs
@@ -199,7 +199,7 @@ impl From<(Real, Real, Real)> for Vec3 {
 
 impl From<Vec3u> for Vec3 {
     fn from(v: Vec3u) -> Self {
-        Self::new(v.x().into(), v.y().into(), v.z().into())
+        Self::new(v.x() as _, v.y() as _, v.z() as _)
     }
 }
 
diff --git a/src/geom/vecu.rs b/src/geom/vecu.rs
index c1da694..b55760b 100644
--- a/src/geom/vecu.rs
+++ b/src/geom/vecu.rs
@@ -1,7 +1,7 @@
 use super::Vec3;
 use std::fmt::{self, Display};
 
-#[derive(Copy, Clone, Default, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Default, Eq, PartialEq)]
 pub struct Vec3u {
     y: u32,
     x: u32,
diff --git a/src/mat.rs b/src/mat.rs
index 464fa64..2f86a82 100644
--- a/src/mat.rs
+++ b/src/mat.rs
@@ -8,6 +8,9 @@ use std::cmp::Ordering;
 
 #[enum_dispatch]
 pub trait Material {
+    fn is_vacuum(&self) -> bool {
+        false
+    }
     /// Return \sigma, the electrical conductivity.
     /// For a vacuum, this is zero. For a perfect conductor, \inf.
     fn conductivity(&self) -> Vec3 {
@@ -43,6 +46,9 @@ impl Static {
 }
 
 impl Material for Static {
+    fn is_vacuum(&self) -> bool {
+        self.conductivity == Default::default()
+    }
     fn conductivity(&self) -> Vec3 {
         self.conductivity
     }
diff --git a/src/meas.rs b/src/meas.rs
index 3b2e877..016d718 100644
--- a/src/meas.rs
+++ b/src/meas.rs
@@ -115,7 +115,7 @@ impl AbstractMeasurement for Energy {
          let f = state.feature_size();
          #[allow(non_snake_case)]
          let dV = f*f*f;
-        let e = state.map_sum(|cell| {
+         let e = state.map_sum(|cell| {
             // E . D is perpetually 0 since we don't model D.
             // All potential energy is in the magnetic field.
             0.5 * cell.h().dot(cell.b()) * dV
diff --git a/src/render.rs b/src/render.rs
index bbc602b..e5b12b8 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -70,9 +70,15 @@ impl<'a> RenderSteps<'a> {
             width = (width as f32 * stretch) as _;
             height = max_height;
         }
-        trace!("rendering at {}x{}", width, height);
+        trace!("rendering at {}x{} with z={}", width, height, z);
         let mut me = Self::new(state, measurements, width, height, z);
-        me.render_scalar_field(10.0,  false, 2, |cell| cell.mat().conductivity().mag());
+        me.render_scalar_field(10.0,  false, 2, |cell| {
+            cell.mat().conductivity().mag() + if cell.mat().is_vacuum() {
+                0.0
+            } else {
+                5.0
+            }
+        });
         me.render_scalar_field(100.0, true,  0, |cell| cell.mat().m().mag());
         if false {
             me.render_b_z_field();