diff --git a/index.html b/index.html
index 3295298..c7f8f79 100644
--- a/index.html
+++ b/index.html
@@ -68,10 +68,16 @@ aside > container > div {
 <body>
 <aside class="js">
 <container>
-<div style="width:100%">
+<div>
 <button onclick="toggle_physics()">Physics</button>
 <input id="checkbox-physics" type="checkbox" disabled>
 </div>
+<form style="display:grid;grid-template-columns:1fr auto;gap:5px;">
+<input id="input-charge" type="range" min="-1.5" max="1.5" step="0.25"
+  style="min-width:100px;max-width:128px;"
+  oninput="set_charge(-Math.pow(10, input_charge.valueAsNumber))">
+<input type="reset">
+</form>
 <div>
 <button onclick="toggle_skeleton()">Skeleton</button>
 <input id="checkbox-skeleton" type="checkbox" disabled>
@@ -108,6 +114,10 @@ Source code: <a href="https://git.atomic.garden/root/sphere-electrons">https://g
 </noscript>
 <script src="thomson-problem.js"></script>
 <script src="sketch.js"></script>
-<script>document.querySelectorAll(".js").forEach(e => e.style = "display:initial");</script>
+<script>
+document.querySelectorAll(".js").forEach(e => e.style = "display:initial");
+const input_charge = document.getElementById("input-charge");
+input_charge.oninput();
+</script>
 </body>
 </html>
diff --git a/sketch.js b/sketch.js
index b8f9c3d..a13c15a 100644
--- a/sketch.js
+++ b/sketch.js
@@ -319,3 +319,10 @@ function toggle_polytope() {
   polytope = !polytope;
   checkbox_polytope.checked = polytope;
 }
+
+function set_charge(value) {
+  charge = value;
+  for (let particle of particles) {
+    particle.charge = value;
+  }
+}
