diff --git a/content/zettels/gravity_compensation.md b/content/zettels/gravity_compensation.md
index b85c291..ee27dff 100644
--- a/content/zettels/gravity_compensation.md
+++ b/content/zettels/gravity_compensation.md
@@ -18,6 +18,9 @@ Tags
 (<a href="#citeproc_bib_item_1">Hol, Lomonova, and Vandenput 2006</a>)
 
 
+## Simple Spring {#simple-spring}
+
+
 ## Constant force spring {#constant-force-spring}
 
 
@@ -26,9 +29,14 @@ Tags
 As the mass / position of the load may change during operation, a variable gravity compensation mechanism is very useful.
 
 
+## Commercial products {#commercial-products}
+
+<https://linmot.com/blog/floating-weights-with-magspring/>
+
+
 ## Bibliography {#bibliography}
 
 <style>.csl-entry{text-indent: -1.5em; margin-left: 1.5em;}</style><div class="csl-bib-body">
   <div class="csl-entry"><a id="citeproc_bib_item_1"></a>Hol, S.A.J., E. Lomonova, and A.J.A. Vandenput. 2006. “Design of a Magnetic Gravity Compensation System.” <i>Precision Engineering</i> 30 (3): 265–73. doi:<a href="https://doi.org/10.1016/j.precisioneng.2005.09.005">10.1016/j.precisioneng.2005.09.005</a>.</div>
-  <div class="csl-entry"><a id="citeproc_bib_item_2"></a>Yoshioka, Hayato, Hidenori Shinno, Jiang Zhu, and Manabu Uchiumi. 2017. “A Newly Developed Zero-Gravity Vertical Motion Mechanism for Precision Machining.” <i>Cirp Annals</i> 66 (1): 389–92. doi:<a href="https://doi.org/10.1016/j.cirp.2017.04.057">10.1016/j.cirp.2017.04.057</a>.</div>
+  <div class="csl-entry"><a id="citeproc_bib_item_2"></a>Yoshioka, Hayato, Hidenori Shinno, Jiang Zhu, and Manabu Uchiumi. 2017. “A Newly Developed Zero-Gravity Vertical Motion Mechanism for Precision Machining.” <i>CIRP Annals</i> 66 (1): 389–92. doi:<a href="https://doi.org/10.1016/j.cirp.2017.04.057">10.1016/j.cirp.2017.04.057</a>.</div>
 </div>